From 6ac392815628f317fcfdca1a39df00b9cc4ebc8b Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Wed, 3 May 2023 13:18:42 +0200 Subject: [PATCH] fs: allow to mount beneath top mount MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Various distributions are adding or are in the process of adding support for system extensions and in the future configuration extensions through various tools. A more detailed explanation on system and configuration extensions can be found on the manpage which is listed below at [1]. System extension images may – dynamically at runtime — extend the /usr/ and /opt/ directory hierarchies with additional files. This is particularly useful on immutable system images where a /usr/ and/or /opt/ hierarchy residing on a read-only file system shall be extended temporarily at runtime without making any persistent modifications. When one or more system extension images are activated, their /usr/ and /opt/ hierarchies are combined via overlayfs with the same hierarchies of the host OS, and the host /usr/ and /opt/ overmounted with it ("merging"). When they are deactivated, the mount point is disassembled — again revealing the unmodified original host version of the hierarchy ("unmerging"). Merging thus makes the extension's resources suddenly appear below the /usr/ and /opt/ hierarchies as if they were included in the base OS image itself. Unmerging makes them disappear again, leaving in place only the files that were shipped with the base OS image itself. System configuration images are similar but operate on directories containing system or service configuration. On nearly all modern distributions mount propagation plays a crucial role and the rootfs of the OS is a shared mount in a peer group (usually with peer group id 1): TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID / / ext4 shared:1 29 1 On such systems all services and containers run in a separate mount namespace and are pivot_root()ed into their rootfs. A separate mount namespace is almost always used as it is the minimal isolation mechanism services have. But usually they are even much more isolated up to the point where they almost become indistinguishable from containers. Mount propagation again plays a crucial role here. The rootfs of all these services is a slave mount to the peer group of the host rootfs. This is done so the service will receive mount propagation events from the host when certain files or directories are updated. In addition, the rootfs of each service, container, and sandbox is also a shared mount in its separate peer group: TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID / / ext4 shared:24 master:1 71 47 For people not too familiar with mount propagation, the master:1 means that this is a slave mount to peer group 1. Which as one can see is the host rootfs as indicated by shared:1 above. The shared:24 indicates that the service rootfs is a shared mount in a separate peer group with peer group id 24. A service may run other services. Such nested services will also have a rootfs mount that is a slave to the peer group of the outer service rootfs mount. For containers things are just slighly different. A container's rootfs isn't a slave to the service's or host rootfs' peer group. The rootfs mount of a container is simply a shared mount in its own peer group: TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID /home/ubuntu/debian-tree / ext4 shared:99 61 60 So whereas services are isolated OS components a container is treated like a separate world and mount propagation into it is restricted to a single well known mount that is a slave to the peer group of the shared mount /run on the host: TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID /propagate/debian-tree /run/host/incoming tmpfs master:5 71 68 Here, the master:5 indicates that this mount is a slave to the peer group with peer group id 5. This allows to propagate mounts into the container and served as a workaround for not being able to insert mounts into mount namespaces directly. But the new mount api does support inserting mounts directly. For the interested reader the blogpost in [2] might be worth reading where I explain the old and the new approach to inserting mounts into mount namespaces. Containers of course, can themselves be run as services. They often run full systems themselves which means they again run services and containers with the exact same propagation settings explained above. The whole system is designed so that it can be easily updated, including all services in various fine-grained ways without having to enter every single service's mount namespace which would be prohibitively expensive. The mount propagation layout has been carefully chosen so it is possible to propagate updates for system extensions and configurations from the host into all services. The simplest model to update the whole system is to mount on top of /usr, /opt, or /etc on the host. The new mount on /usr, /opt, or /etc will then propagate into every service. This works cleanly the first time. However, when the system is updated multiple times it becomes necessary to unmount the first update on /opt, /usr, /etc and then propagate the new update. But this means, there's an interval where the old base system is accessible. This has to be avoided to protect against downgrade attacks. The vfs already exposes a mechanism to userspace whereby mounts can be mounted beneath an existing mount. Such mounts are internally referred to as "tucked". The patch series exposes the ability to mount beneath a top mount through the new MOVE_MOUNT_BENEATH flag for the move_mount() system call. This allows userspace to seamlessly upgrade mounts. After this series the only thing that will have changed is that mounting beneath an existing mount can be done explicitly instead of just implicitly. Today, there are two scenarios where a mount can be mounted beneath an existing mount instead of on top of it: (1) When a service or container is started in a new mount namespace and pivot_root()s into its new rootfs. The way this is done is by mounting the new rootfs beneath the old rootfs: fd_newroot = open("/var/lib/machines/fedora", ...); fd_oldroot = open("/", ...); fchdir(fd_newroot); pivot_root(".", "."); After the pivot_root(".", ".") call the new rootfs is mounted beneath the old rootfs which can then be unmounted to reveal the underlying mount: fchdir(fd_oldroot); umount2(".", MNT_DETACH); Since pivot_root() moves the caller into a new rootfs no mounts must be propagated out of the new rootfs as a consequence of the pivot_root() call. Thus, the mounts cannot be shared. (2) When a mount is propagated to a mount that already has another mount mounted on the same dentry. The easiest example for this is to create a new mount namespace. The following commands will create a mount namespace where the rootfs mount / will be a slave to the peer group of the host rootfs / mount's peer group. IOW, it will receive propagation from the host: mount --make-shared / unshare --mount --propagation=slave Now a new mount on the /mnt dentry in that mount namespace is created. (As it can be confusing it should be spelled out that the tmpfs mount on the /mnt dentry that was just created doesn't propagate back to the host because the rootfs mount / of the mount namespace isn't a peer of the host rootfs.): mount -t tmpfs tmpfs /mnt TARGET SOURCE FSTYPE PROPAGATION └─/mnt tmpfs tmpfs Now another terminal in the host mount namespace can observe that the mount indeed hasn't propagated back to into the host mount namespace. A new mount can now be created on top of the /mnt dentry with the rootfs mount / as its parent: mount --bind /opt /mnt TARGET SOURCE FSTYPE PROPAGATION └─/mnt /dev/sda2[/opt] ext4 shared:1 The mount namespace that was created earlier can now observe that the bind mount created on the host has propagated into it: TARGET SOURCE FSTYPE PROPAGATION └─/mnt /dev/sda2[/opt] ext4 master:1 └─/mnt tmpfs tmpfs But instead of having been mounted on top of the tmpfs mount at the /mnt dentry the /opt mount has been mounted on top of the rootfs mount at the /mnt dentry. And the tmpfs mount has been remounted on top of the propagated /opt mount at the /opt dentry. So in other words, the propagated mount has been mounted beneath the preexisting mount in that mount namespace. Mount namespaces make this easy to illustrate but it's also easy to mount beneath an existing mount in the same mount namespace (The following example assumes a shared rootfs mount / with peer group id 1): mount --bind /opt /opt TARGET SOURCE FSTYPE MNT_ID PARENT_ID PROPAGATION └─/opt /dev/sda2[/opt] ext4 188 29 shared:1 If another mount is mounted on top of the /opt mount at the /opt dentry: mount --bind /tmp /opt The following clunky mount tree will result: TARGET SOURCE FSTYPE MNT_ID PARENT_ID PROPAGATION └─/opt /dev/sda2[/tmp] ext4 405 29 shared:1 └─/opt /dev/sda2[/opt] ext4 188 405 shared:1 └─/opt /dev/sda2[/tmp] ext4 404 188 shared:1 The /tmp mount is mounted beneath the /opt mount and another copy is mounted on top of the /opt mount. This happens because the rootfs / and the /opt mount are shared mounts in the same peer group. When the new /tmp mount is supposed to be mounted at the /opt dentry then the /tmp mount first propagates to the root mount at the /opt dentry. But there already is the /opt mount mounted at the /opt dentry. So the old /opt mount at the /opt dentry will be mounted on top of the new /tmp mount at the /tmp dentry, i.e. @opt->mnt_parent is @tmp and @opt->mnt_mountpoint is /tmp (Note that @opt->mnt_root is /opt which is what shows up as /opt under SOURCE). So again, a mount will be mounted beneath a preexisting mount. (Fwiw, a few iterations of mount --bind /opt /opt in a loop on a shared rootfs is a good example of what could be referred to as mount explosion.) The main point is that such mounts allows userspace to umount a top mount and reveal an underlying mount. So for example, umounting the tmpfs mount on /mnt that was created in example (1) using mount namespaces reveals the /opt mount which was mounted beneath it. In (2) where a mount was mounted beneath the top mount in the same mount namespace unmounting the top mount would unmount both the top mount and the mount beneath. In the process the original mount would be remounted on top of the rootfs mount / at the /opt dentry again. This again, is a result of mount propagation only this time it's umount propagation. However, this can be avoided by simply making the parent mount / of the @opt mount a private or slave mount. Then the top mount and the original mount can be unmounted to reveal the mount beneath. These two examples are fairly arcane and are merely added to make it clear how mount propagation has effects on current and future features. More common use-cases will just be things like: mount -t btrfs /dev/sdA /mnt mount -t xfs /dev/sdB --beneath /mnt umount /mnt after which we'll have updated from a btrfs filesystem to a xfs filesystem without ever revealing the underlying mountpoint. The crux is that the proposed mechanism already exists and that it is so powerful as to cover cases where mounts are supposed to be updated with new versions. Crucially, it offers an important flexibility. Namely that updates to a system may either be forced or can be delayed and the umount of the top mount be left to a service if it is a cooperative one. This adds a new flag to move_mount() that allows to explicitly move a beneath the top mount adhering to the following semantics: * Mounts cannot be mounted beneath the rootfs. This restriction encompasses the rootfs but also chroots via chroot() and pivot_root(). To mount a mount beneath the rootfs or a chroot, pivot_root() can be used as illustrated above. * The source mount must be a private mount to force the kernel to allocate a new, unused peer group id. This isn't a required restriction but a voluntary one. It avoids repeating a semantical quirk that already exists today. If bind mounts which already have a peer group id are inserted into mount trees that have the same peer group id this can cause a lot of mount propagation events to be generated (For example, consider running mount --bind /opt /opt in a loop where the parent mount is a shared mount.). * Avoid getting rid of the top mount in the kernel. Cooperative services need to be able to unmount the top mount themselves. This also avoids a good deal of additional complexity. The umount would have to be propagated which would be another rather expensive operation. So namespace_lock() and lock_mount_hash() would potentially have to be held for a long time for both a mount and umount propagation. That should be avoided. * The path to mount beneath must be mounted and attached. * The top mount and its parent must be in the caller's mount namespace and the caller must be able to mount in that mount namespace. * The caller must be able to unmount the top mount to prove that they could reveal the underlying mount. * The propagation tree is calculated based on the destination mount's parent mount and the destination mount's mountpoint on the parent mount. Of course, if the parent of the destination mount and the destination mount are shared mounts in the same peer group and the mountpoint of the new mount to be mounted is a subdir of their ->mnt_root then both will receive a mount of /opt. That's probably easier to understand with an example. Assuming a standard shared rootfs /: mount --bind /opt /opt mount --bind /tmp /opt will cause the same mount tree as: mount --bind /opt /opt mount --beneath /tmp /opt because both / and /opt are shared mounts/peers in the same peer group and the /opt dentry is a subdirectory of both the parent's and the child's ->mnt_root. If a mount tree like that is created it almost always is an accident or abuse of mount propagation. Realistically what most people probably mean in this scenarios is: mount --bind /opt /opt mount --make-private /opt mount --make-shared /opt This forces the allocation of a new separate peer group for the /opt mount. Aferwards a mount --bind or mount --beneath actually makes sense as the / and /opt mount belong to different peer groups. Before that it's likely just confusion about what the user wanted to achieve. * Refuse MOVE_MOUNT_BENEATH if: (1) the @mnt_from has been overmounted in between path resolution and acquiring @namespace_sem when locking @mnt_to. This avoids the proliferation of shadow mounts. (2) if @to_mnt is moved to a different mountpoint while acquiring @namespace_sem to lock @to_mnt. (3) if @to_mnt is unmounted while acquiring @namespace_sem to lock @to_mnt. (4) if the parent of the target mount propagates to the target mount at the same mountpoint. This would mean mounting @mnt_from on @mnt_to->mnt_parent and then propagating a copy @c of @mnt_from onto @mnt_to. This defeats the whole purpose of mounting @mnt_from beneath @mnt_to. (5) if the parent mount @mnt_to->mnt_parent propagates to @mnt_from at the same mountpoint. If @mnt_to->mnt_parent propagates to @mnt_from this would mean propagating a copy @c of @mnt_from on top of @mnt_from. Afterwards @mnt_from would be mounted on top of @mnt_to->mnt_parent and @mnt_to would be unmounted from @mnt->mnt_parent and remounted on @mnt_from. But since @c is already mounted on @mnt_from, @mnt_to would ultimately be remounted on top of @c. Afterwards, @mnt_from would be covered by a copy @c of @mnt_from and @c would be covered by @mnt_from itself. This defeats the whole purpose of mounting @mnt_from beneath @mnt_to. Cases (1) to (3) are required as they deal with races that would cause bugs or unexpected behavior for users. Cases (4) and (5) refuse semantical quirks that would not be a bug but would cause weird mount trees to be created. While they can already be created via other means (mount --bind /opt /opt x n) there's no reason to repeat past mistakes in new features. Link: https://man7.org/linux/man-pages/man8/systemd-sysext.8.html [1] Link: https://brauner.io/2023/02/28/mounting-into-mount-namespaces.html [2] Link: https://github.com/flatcar/sysext-bakery Link: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Support_Phase_1 Link: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Support_Phase_2 Link: https://github.com/systemd/systemd/pull/26013 Reviewed-by: Seth Forshee (DigitalOcean) Message-Id: <20230202-fs-move-mount-replace-v4-4-98f3d80d7eaa@kernel.org> Signed-off-by: Christian Brauner --- fs/namespace.c | 352 ++++++++++++++++++++++++++++++++----- fs/pnode.c | 42 ++++- fs/pnode.h | 3 + include/uapi/linux/mount.h | 3 +- 4 files changed, 350 insertions(+), 50 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index f91a60e6039d..382edc76aee7 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -926,6 +926,33 @@ void mnt_set_mountpoint(struct mount *mnt, hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list); } +/** + * mnt_set_mountpoint_beneath - mount a mount beneath another one + * + * @new_parent: the source mount + * @top_mnt: the mount beneath which @new_parent is mounted + * @new_mp: the new mountpoint of @top_mnt on @new_parent + * + * Remove @top_mnt from its current mountpoint @top_mnt->mnt_mp and + * parent @top_mnt->mnt_parent and mount it on top of @new_parent at + * @new_mp. And mount @new_parent on the old parent and old + * mountpoint of @top_mnt. + * + * Context: This function expects namespace_lock() and lock_mount_hash() + * to have been acquired in that order. + */ +static void mnt_set_mountpoint_beneath(struct mount *new_parent, + struct mount *top_mnt, + struct mountpoint *new_mp) +{ + struct mount *old_top_parent = top_mnt->mnt_parent; + struct mountpoint *old_top_mp = top_mnt->mnt_mp; + + mnt_set_mountpoint(old_top_parent, old_top_mp, new_parent); + mnt_change_mountpoint(new_parent, new_mp, top_mnt); +} + + static void __attach_mnt(struct mount *mnt, struct mount *parent) { hlist_add_head_rcu(&mnt->mnt_hash, @@ -933,15 +960,42 @@ static void __attach_mnt(struct mount *mnt, struct mount *parent) list_add_tail(&mnt->mnt_child, &parent->mnt_mounts); } -/* - * vfsmount lock must be held for write +/** + * attach_mnt - mount a mount, attach to @mount_hashtable and parent's + * list of child mounts + * @parent: the parent + * @mnt: the new mount + * @mp: the new mountpoint + * @beneath: whether to mount @mnt beneath or on top of @parent + * + * If @beneath is false, mount @mnt at @mp on @parent. Then attach @mnt + * to @parent's child mount list and to @mount_hashtable. + * + * If @beneath is true, remove @mnt from its current parent and + * mountpoint and mount it on @mp on @parent, and mount @parent on the + * old parent and old mountpoint of @mnt. Finally, attach @parent to + * @mnt_hashtable and @parent->mnt_parent->mnt_mounts. + * + * Note, when __attach_mnt() is called @mnt->mnt_parent already points + * to the correct parent. + * + * Context: This function expects namespace_lock() and lock_mount_hash() + * to have been acquired in that order. */ -static void attach_mnt(struct mount *mnt, - struct mount *parent, - struct mountpoint *mp) +static void attach_mnt(struct mount *mnt, struct mount *parent, + struct mountpoint *mp, bool beneath) { - mnt_set_mountpoint(parent, mp, mnt); - __attach_mnt(mnt, parent); + if (beneath) + mnt_set_mountpoint_beneath(mnt, parent, mp); + else + mnt_set_mountpoint(parent, mp, mnt); + /* + * Note, @mnt->mnt_parent has to be used. If @mnt was mounted + * beneath @parent then @mnt will need to be attached to + * @parent's old parent, not @parent. IOW, @mnt->mnt_parent + * isn't the same mount as @parent. + */ + __attach_mnt(mnt, mnt->mnt_parent); } void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct mount *mnt) @@ -953,7 +1007,7 @@ void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct m hlist_del_init(&mnt->mnt_mp_list); hlist_del_init_rcu(&mnt->mnt_hash); - attach_mnt(mnt, parent, mp); + attach_mnt(mnt, parent, mp, false); put_mountpoint(old_mp); mnt_add_count(old_parent, -1); @@ -1954,7 +2008,7 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry, goto out; lock_mount_hash(); list_add_tail(&q->mnt_list, &res->mnt_list); - attach_mnt(q, parent, p->mnt_mp); + attach_mnt(q, parent, p->mnt_mp, false); unlock_mount_hash(); } } @@ -2163,12 +2217,17 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt) return 0; } -/* - * @source_mnt : mount tree to be attached - * @nd : place the mount tree @source_mnt is attached - * @parent_nd : if non-null, detach the source_mnt from its parent and - * store the parent mount and mountpoint dentry. - * (done when source_mnt is moved) +enum mnt_tree_flags_t { + MNT_TREE_MOVE = BIT(0), + MNT_TREE_BENEATH = BIT(1), +}; + +/** + * attach_recursive_mnt - attach a source mount tree + * @source_mnt: mount tree to be attached + * @top_mnt: mount that @source_mnt will be mounted on or mounted beneath + * @dest_mp: the mountpoint @source_mnt will be mounted at + * @flags: modify how @source_mnt is supposed to be attached * * NOTE: in the table below explains the semantics when a source mount * of a given type is attached to a destination mount of a given type. @@ -2225,22 +2284,28 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt) * applied to each mount in the tree. * Must be called without spinlocks held, since this function can sleep * in allocations. + * + * Context: The function expects namespace_lock() to be held. + * Return: If @source_mnt was successfully attached 0 is returned. + * Otherwise a negative error code is returned. */ static int attach_recursive_mnt(struct mount *source_mnt, - struct mount *dest_mnt, - struct mountpoint *dest_mp, - bool moving) + struct mount *top_mnt, + struct mountpoint *dest_mp, + enum mnt_tree_flags_t flags) { struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns; HLIST_HEAD(tree_list); - struct mnt_namespace *ns = dest_mnt->mnt_ns; + struct mnt_namespace *ns = top_mnt->mnt_ns; struct mountpoint *smp; - struct mount *child, *p; + struct mount *child, *dest_mnt, *p; struct hlist_node *n; - int err; + int err = 0; + bool moving = flags & MNT_TREE_MOVE, beneath = flags & MNT_TREE_BENEATH; - /* Preallocate a mountpoint in case the new mounts need - * to be tucked under other mounts. + /* + * Preallocate a mountpoint in case the new mounts need to be + * mounted beneath mounts on the same mountpoint. */ smp = get_mountpoint(source_mnt->mnt.mnt_root); if (IS_ERR(smp)) @@ -2253,29 +2318,41 @@ static int attach_recursive_mnt(struct mount *source_mnt, goto out; } + if (beneath) + dest_mnt = top_mnt->mnt_parent; + else + dest_mnt = top_mnt; + if (IS_MNT_SHARED(dest_mnt)) { err = invent_group_ids(source_mnt, true); if (err) goto out; err = propagate_mnt(dest_mnt, dest_mp, source_mnt, &tree_list); - lock_mount_hash(); - if (err) - goto out_cleanup_ids; + } + lock_mount_hash(); + if (err) + goto out_cleanup_ids; + + if (IS_MNT_SHARED(dest_mnt)) { for (p = source_mnt; p; p = next_mnt(p, source_mnt)) set_mnt_shared(p); - } else { - lock_mount_hash(); } + if (moving) { + if (beneath) + dest_mp = smp; unhash_mnt(source_mnt); - attach_mnt(source_mnt, dest_mnt, dest_mp); + attach_mnt(source_mnt, top_mnt, dest_mp, beneath); touch_mnt_namespace(source_mnt->mnt_ns); } else { if (source_mnt->mnt_ns) { /* move from anon - the caller will destroy */ list_del_init(&source_mnt->mnt_ns->list); } - mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt); + if (beneath) + mnt_set_mountpoint_beneath(source_mnt, top_mnt, smp); + else + mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt); commit_tree(source_mnt); } @@ -2315,28 +2392,80 @@ static int attach_recursive_mnt(struct mount *source_mnt, return err; } -static struct mountpoint *lock_mount(struct path *path) +/** + * do_lock_mount - lock mount and mountpoint + * @path: target path + * @beneath: whether the intention is to mount beneath @path + * + * Follow the mount stack on @path until the top mount @mnt is found. If + * the initial @path->{mnt,dentry} is a mountpoint lookup the first + * mount stacked on top of it. Then simply follow @{mnt,mnt->mnt_root} + * until nothing is stacked on top of it anymore. + * + * Acquire the inode_lock() on the top mount's ->mnt_root to protect + * against concurrent removal of the new mountpoint from another mount + * namespace. + * + * If @beneath is requested, acquire inode_lock() on @mnt's mountpoint + * @mp on @mnt->mnt_parent must be acquired. This protects against a + * concurrent unlink of @mp->mnt_dentry from another mount namespace + * where @mnt doesn't have a child mount mounted @mp. A concurrent + * removal of @mnt->mnt_root doesn't matter as nothing will be mounted + * on top of it for @beneath. + * + * In addition, @beneath needs to make sure that @mnt hasn't been + * unmounted or moved from its current mountpoint in between dropping + * @mount_lock and acquiring @namespace_sem. For the !@beneath case @mnt + * being unmounted would be detected later by e.g., calling + * check_mnt(mnt) in the function it's called from. For the @beneath + * case however, it's useful to detect it directly in do_lock_mount(). + * If @mnt hasn't been unmounted then @mnt->mnt_mountpoint still points + * to @mnt->mnt_mp->m_dentry. But if @mnt has been unmounted it will + * point to @mnt->mnt_root and @mnt->mnt_mp will be NULL. + * + * Return: Either the target mountpoint on the top mount or the top + * mount's mountpoint. + */ +static struct mountpoint *do_lock_mount(struct path *path, bool beneath) { - struct vfsmount *mnt; + struct vfsmount *mnt = path->mnt; struct dentry *dentry; - struct mountpoint *mp; + struct mountpoint *mp = ERR_PTR(-ENOENT); for (;;) { - dentry = path->dentry; + struct mount *m; + + if (beneath) { + m = real_mount(mnt); + read_seqlock_excl(&mount_lock); + dentry = dget(m->mnt_mountpoint); + read_sequnlock_excl(&mount_lock); + } else { + dentry = path->dentry; + } + inode_lock(dentry->d_inode); if (unlikely(cant_mount(dentry))) { inode_unlock(dentry->d_inode); - return ERR_PTR(-ENOENT); + goto out; } namespace_lock(); + if (beneath && (!is_mounted(mnt) || m->mnt_mountpoint != dentry)) { + namespace_unlock(); + inode_unlock(dentry->d_inode); + goto out; + } + mnt = lookup_mnt(path); if (likely(!mnt)) break; namespace_unlock(); inode_unlock(dentry->d_inode); + if (beneath) + dput(dentry); path_put(path); path->mnt = mnt; path->dentry = dget(mnt->mnt_root); @@ -2348,9 +2477,18 @@ static struct mountpoint *lock_mount(struct path *path) inode_unlock(dentry->d_inode); } +out: + if (beneath) + dput(dentry); + return mp; } +static inline struct mountpoint *lock_mount(struct path *path) +{ + return do_lock_mount(path, false); +} + static void unlock_mount(struct mountpoint *where) { struct dentry *dentry = where->m_dentry; @@ -2372,7 +2510,7 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp) d_is_dir(mnt->mnt.mnt_root)) return -ENOTDIR; - return attach_recursive_mnt(mnt, p, mp, false); + return attach_recursive_mnt(mnt, p, mp, 0); } /* @@ -2854,7 +2992,110 @@ out: return err; } -static int do_move_mount(struct path *old_path, struct path *new_path) +/** + * path_overmounted - check if path is overmounted + * @path: path to check + * + * Check if path is overmounted, i.e., if there's a mount on top of + * @path->mnt with @path->dentry as mountpoint. + * + * Context: This function expects namespace_lock() to be held. + * Return: If path is overmounted true is returned, false if not. + */ +static inline bool path_overmounted(const struct path *path) +{ + rcu_read_lock(); + if (unlikely(__lookup_mnt(path->mnt, path->dentry))) { + rcu_read_unlock(); + return true; + } + rcu_read_unlock(); + return false; +} + +/** + * can_move_mount_beneath - check that we can mount beneath the top mount + * @from: mount to mount beneath + * @to: mount under which to mount + * + * - Make sure that @to->dentry is actually the root of a mount under + * which we can mount another mount. + * - Make sure that nothing can be mounted beneath the caller's current + * root or the rootfs of the namespace. + * - Make sure that the caller can unmount the topmost mount ensuring + * that the caller could reveal the underlying mountpoint. + * - Ensure that nothing has been mounted on top of @from before we + * grabbed @namespace_sem to avoid creating pointless shadow mounts. + * - Prevent mounting beneath a mount if the propagation relationship + * between the source mount, parent mount, and top mount would lead to + * nonsensical mount trees. + * + * Context: This function expects namespace_lock() to be held. + * Return: On success 0, and on error a negative error code is returned. + */ +static int can_move_mount_beneath(const struct path *from, + const struct path *to, + const struct mountpoint *mp) +{ + struct mount *mnt_from = real_mount(from->mnt), + *mnt_to = real_mount(to->mnt), + *parent_mnt_to = mnt_to->mnt_parent; + + if (!mnt_has_parent(mnt_to)) + return -EINVAL; + + if (!path_mounted(to)) + return -EINVAL; + + if (IS_MNT_LOCKED(mnt_to)) + return -EINVAL; + + /* Avoid creating shadow mounts during mount propagation. */ + if (path_overmounted(from)) + return -EINVAL; + + /* + * Mounting beneath the rootfs only makes sense when the + * semantics of pivot_root(".", ".") are used. + */ + if (&mnt_to->mnt == current->fs->root.mnt) + return -EINVAL; + if (parent_mnt_to == current->nsproxy->mnt_ns->root) + return -EINVAL; + + for (struct mount *p = mnt_from; mnt_has_parent(p); p = p->mnt_parent) + if (p == mnt_to) + return -EINVAL; + + /* + * If the parent mount propagates to the child mount this would + * mean mounting @mnt_from on @mnt_to->mnt_parent and then + * propagating a copy @c of @mnt_from on top of @mnt_to. This + * defeats the whole purpose of mounting beneath another mount. + */ + if (propagation_would_overmount(parent_mnt_to, mnt_to, mp)) + return -EINVAL; + + /* + * If @mnt_to->mnt_parent propagates to @mnt_from this would + * mean propagating a copy @c of @mnt_from on top of @mnt_from. + * Afterwards @mnt_from would be mounted on top of + * @mnt_to->mnt_parent and @mnt_to would be unmounted from + * @mnt->mnt_parent and remounted on @mnt_from. But since @c is + * already mounted on @mnt_from, @mnt_to would ultimately be + * remounted on top of @c. Afterwards, @mnt_from would be + * covered by a copy @c of @mnt_from and @c would be covered by + * @mnt_from itself. This defeats the whole purpose of mounting + * @mnt_from beneath @mnt_to. + */ + if (propagation_would_overmount(parent_mnt_to, mnt_from, mp)) + return -EINVAL; + + return 0; +} + +static int do_move_mount(struct path *old_path, struct path *new_path, + bool beneath) { struct mnt_namespace *ns; struct mount *p; @@ -2863,8 +3104,9 @@ static int do_move_mount(struct path *old_path, struct path *new_path) struct mountpoint *mp, *old_mp; int err; bool attached; + enum mnt_tree_flags_t flags = 0; - mp = lock_mount(new_path); + mp = do_lock_mount(new_path, beneath); if (IS_ERR(mp)) return PTR_ERR(mp); @@ -2872,6 +3114,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path) p = real_mount(new_path->mnt); parent = old->mnt_parent; attached = mnt_has_parent(old); + if (attached) + flags |= MNT_TREE_MOVE; old_mp = old->mnt_mp; ns = old->mnt_ns; @@ -2902,6 +3146,17 @@ static int do_move_mount(struct path *old_path, struct path *new_path) */ if (attached && IS_MNT_SHARED(parent)) goto out; + + if (beneath) { + err = can_move_mount_beneath(old_path, new_path, mp); + if (err) + goto out; + + err = -EINVAL; + p = p->mnt_parent; + flags |= MNT_TREE_BENEATH; + } + /* * Don't move a mount tree containing unbindable mounts to a destination * mount which is shared. @@ -2915,8 +3170,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path) if (p == old) goto out; - err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp, - attached); + err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp, flags); if (err) goto out; @@ -2948,7 +3202,7 @@ static int do_move_mount_old(struct path *path, const char *old_name) if (err) return err; - err = do_move_mount(&old_path, path); + err = do_move_mount(&old_path, path, false); path_put(&old_path); return err; } @@ -3114,13 +3368,10 @@ int finish_automount(struct vfsmount *m, const struct path *path) err = -ENOENT; goto discard_locked; } - rcu_read_lock(); - if (unlikely(__lookup_mnt(path->mnt, dentry))) { - rcu_read_unlock(); + if (path_overmounted(path)) { err = 0; goto discard_locked; } - rcu_read_unlock(); mp = get_mountpoint(dentry); if (IS_ERR(mp)) { err = PTR_ERR(mp); @@ -3812,6 +4063,10 @@ SYSCALL_DEFINE5(move_mount, if (flags & ~MOVE_MOUNT__MASK) return -EINVAL; + if ((flags & (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP)) == + (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP)) + return -EINVAL; + /* If someone gives a pathname, they aren't permitted to move * from an fd that requires unmount as we can't get at the flag * to clear it afterwards. @@ -3841,7 +4096,8 @@ SYSCALL_DEFINE5(move_mount, if (flags & MOVE_MOUNT_SET_GROUP) ret = do_set_group(&from_path, &to_path); else - ret = do_move_mount(&from_path, &to_path); + ret = do_move_mount(&from_path, &to_path, + (flags & MOVE_MOUNT_BENEATH)); out_to: path_put(&to_path); @@ -3974,9 +4230,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root, root_mnt->mnt.mnt_flags &= ~MNT_LOCKED; } /* mount old root on put_old */ - attach_mnt(root_mnt, old_mnt, old_mp); + attach_mnt(root_mnt, old_mnt, old_mp, false); /* mount new_root on / */ - attach_mnt(new_mnt, root_parent, root_mp); + attach_mnt(new_mnt, root_parent, root_mp, false); mnt_add_count(root_parent, -1); touch_mnt_namespace(current->nsproxy->mnt_ns); /* A moved mount should not expire automatically */ diff --git a/fs/pnode.c b/fs/pnode.c index 3cede8b18c8b..e4d0340393d5 100644 --- a/fs/pnode.c +++ b/fs/pnode.c @@ -216,7 +216,7 @@ static struct mount *next_group(struct mount *m, struct mount *origin) static struct mount *last_dest, *first_source, *last_source, *dest_master; static struct hlist_head *list; -static inline bool peers(struct mount *m1, struct mount *m2) +static inline bool peers(const struct mount *m1, const struct mount *m2) { return m1->mnt_group_id == m2->mnt_group_id && m1->mnt_group_id; } @@ -354,6 +354,46 @@ static inline int do_refcount_check(struct mount *mnt, int count) return mnt_get_count(mnt) > count; } +/** + * propagation_would_overmount - check whether propagation from @from + * would overmount @to + * @from: shared mount + * @to: mount to check + * @mp: future mountpoint of @to on @from + * + * If @from propagates mounts to @to, @from and @to must either be peers + * or one of the masters in the hierarchy of masters of @to must be a + * peer of @from. + * + * If the root of the @to mount is equal to the future mountpoint @mp of + * the @to mount on @from then @to will be overmounted by whatever is + * propagated to it. + * + * Context: This function expects namespace_lock() to be held and that + * @mp is stable. + * Return: If @from overmounts @to, true is returned, false if not. + */ +bool propagation_would_overmount(const struct mount *from, + const struct mount *to, + const struct mountpoint *mp) +{ + if (!IS_MNT_SHARED(from)) + return false; + + if (IS_MNT_NEW(to)) + return false; + + if (to->mnt.mnt_root != mp->m_dentry) + return false; + + for (const struct mount *m = to; m; m = m->mnt_master) { + if (peers(from, m)) + return true; + } + + return false; +} + /* * check if the mount 'mnt' can be unmounted successfully. * @mnt: the mount to be checked for unmount diff --git a/fs/pnode.h b/fs/pnode.h index 988f1aa9b02a..0b02a6393891 100644 --- a/fs/pnode.h +++ b/fs/pnode.h @@ -53,4 +53,7 @@ struct mount *copy_tree(struct mount *, struct dentry *, int); bool is_path_reachable(struct mount *, struct dentry *, const struct path *root); int count_mounts(struct mnt_namespace *ns, struct mount *mnt); +bool propagation_would_overmount(const struct mount *from, + const struct mount *to, + const struct mountpoint *mp); #endif /* _LINUX_PNODE_H */ diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index 4d93967f8aea..8eb0d7b758d2 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -74,7 +74,8 @@ #define MOVE_MOUNT_T_AUTOMOUNTS 0x00000020 /* Follow automounts on to path */ #define MOVE_MOUNT_T_EMPTY_PATH 0x00000040 /* Empty to path permitted */ #define MOVE_MOUNT_SET_GROUP 0x00000100 /* Set sharing group instead */ -#define MOVE_MOUNT__MASK 0x00000177 +#define MOVE_MOUNT_BENEATH 0x00000200 /* Mount beneath top mount */ +#define MOVE_MOUNT__MASK 0x00000377 /* * fsopen() flags.