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core: open /proc/self/mountinfo early to allow mounts over /proc (#5985)

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Enable masking the /proc folder using the 'InaccessiblePaths' unit
option.

This also slightly simplify mounts setup as the bind_remount_recursive
function will only open /proc/self/mountinfo once.

This is based on the suggestion at:
https://lists.freedesktop.org/archives/systemd-devel/2017-April/038634.html
This commit is contained in:
Timothée Ravier 2017-05-19 14:38:40 +02:00 committed by Lennart Poettering
parent 215a2db419
commit ac9de0b379
3 changed files with 31 additions and 8 deletions
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22
src/basic/mount-util.c
View File

@ -317,11 +317,16 @@ static int get_mount_flags(const char *path, unsigned long *flags) {
return 0;
}
int bind_remount_recursive(const char *prefix, bool ro, char **blacklist) {
/* Use this function only if do you have direct access to /proc/self/mountinfo
* and need the caller to open it for you. This is the case when /proc is
* masked or not mounted. Otherwise, use bind_remount_recursive. */
int bind_remount_recursive_with_mountinfo(const char *prefix, bool ro, char **blacklist, FILE *proc_self_mountinfo) {
_cleanup_set_free_free_ Set *done = NULL;
_cleanup_free_ char *cleaned = NULL;
int r;
assert(proc_self_mountinfo);
/* Recursively remount a directory (and all its submounts) read-only or read-write. If the directory is already
* mounted, we reuse the mount and simply mark it MS_BIND|MS_RDONLY (or remove the MS_RDONLY for read-write
* operation). If it isn't we first make it one. Afterwards we apply MS_BIND|MS_RDONLY (or remove MS_RDONLY) to
@ -344,7 +349,6 @@ int bind_remount_recursive(const char *prefix, bool ro, char **blacklist) {
return -ENOMEM;
for (;;) {
_cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
_cleanup_set_free_free_ Set *todo = NULL;
bool top_autofs = false;
char *x;
@ -354,9 +358,7 @@ int bind_remount_recursive(const char *prefix, bool ro, char **blacklist) {
if (!todo)
return -ENOMEM;
proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
if (!proc_self_mountinfo)
return -errno;
rewind(proc_self_mountinfo);
for (;;) {
_cleanup_free_ char *path = NULL, *p = NULL, *type = NULL;
@ -495,6 +497,16 @@ int bind_remount_recursive(const char *prefix, bool ro, char **blacklist) {
}
}
int bind_remount_recursive(const char *prefix, bool ro, char **blacklist) {
_cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
if (!proc_self_mountinfo)
return -errno;
return bind_remount_recursive_with_mountinfo(prefix, ro, blacklist, proc_self_mountinfo);
}
int mount_move_root(const char *path) {
assert(path);

1
src/basic/mount-util.h
View File

@ -36,6 +36,7 @@ int repeat_unmount(const char *path, int flags);
int umount_recursive(const char *target, int flags);
int bind_remount_recursive(const char *prefix, bool ro, char **blacklist);
int bind_remount_recursive_with_mountinfo(const char *prefix, bool ro, char **blacklist, FILE *proc_self_mountinfo);
int mount_move_root(const char *path);

16
src/core/namespace.c
View File

@ -793,13 +793,14 @@ static int apply_mount(
return 0;
}
static int make_read_only(MountEntry *m, char **blacklist) {
static int make_read_only(MountEntry *m, char **blacklist, FILE *proc_self_mountinfo) {
int r = 0;
assert(m);
assert(proc_self_mountinfo);
if (mount_entry_read_only(m))
r = bind_remount_recursive(mount_entry_path(m), true, blacklist);
r = bind_remount_recursive_with_mountinfo(mount_entry_path(m), true, blacklist, proc_self_mountinfo);
else if (m->mode == PRIVATE_DEV) { /* Superblock can be readonly but the submounts can't */
if (mount(NULL, mount_entry_path(m), NULL, MS_REMOUNT|DEV_MOUNT_OPTIONS|MS_RDONLY, NULL) < 0)
r = -errno;
@ -1082,9 +1083,18 @@ int setup_namespace(
}
if (n_mounts > 0) {
_cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
char **blacklist;
unsigned j;
/* Open /proc/self/mountinfo now as it may become unavailable if we mount anything on top of /proc.
* For example, this is the case with the option: 'InaccessiblePaths=/proc' */
proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
if (!proc_self_mountinfo) {
r = -errno;
goto finish;
}
/* First round, add in all special mounts we need */
for (m = mounts; m < mounts + n_mounts; ++m) {
r = apply_mount(root_directory, m, tmp_dir, var_tmp_dir);
@ -1100,7 +1110,7 @@ int setup_namespace(
/* Second round, flip the ro bits if necessary. */
for (m = mounts; m < mounts + n_mounts; ++m) {
r = make_read_only(m, blacklist);
r = make_read_only(m, blacklist, proc_self_mountinfo);
if (r < 0)
goto finish;
}