nspawn: enable FUSE in containers

Linux kernel v4.18 (2018-08-12) added user-namespace support to FUSE, and
bumped the FUSE version to 7.27 (see: da315f6e0398 (Merge tag
'fuse-update-4.18' of
git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse, Linus Torvalds,
2018-06-07).  This means that on such kernels it is safe to enable FUSE in
nspawn containers.

In outer_child(), before calling copy_devnodes(), check the FUSE version to
decide whether enable (>=7.27) or disable (<7.27) FUSE in the container.  We
look at the FUSE version instead of the kernel version in order to enable FUSE
support on older-versioned kernels that may have the mentioned patchset
backported ([as requested by @poettering][1]).  However, I am not sure that
this is safe; user-namespace support is not a documented part of the FUSE
protocol, which is what FUSE_KERNEL_VERSION/FUSE_KERNEL_MINOR_VERSION are meant
to capture.  While the same patchset
 - added FUSE_ABORT_ERROR (which is all that the 7.27 version bump
   is documented as including),
 - bumped FUSE_KERNEL_MINOR_VERSION from 26 to 27, and
 - added user-namespace support
these 3 things are not inseparable; it is conceivable to me that a backport
could include the first 2 of those things and exclude the 3rd; perhaps it would
be safer to check the kernel version.

Do note that our get_fuse_version() function uses the fsopen() family of
syscalls, which were not added until Linux kernel v5.2 (2019-07-07); so if
nothing has been backported, then the minimum kernel version for FUSE-in-nspawn
is actually v5.2, not v4.18.

Pass whether or not to enable FUSE to copy_devnodes(); have copy_devnodes()
copy in /dev/fuse if enabled.

Pass whether or not to enable FUSE back over fd_outer_socket to run_container()
so that it can pass that to append_machine_properties() (via either
register_machine() or allocate_scope()); have append_machine_properties()
append "DeviceAllow=/dev/fuse rw" if enabled.

For testing, simply check that /dev/fuse can be opened for reading and writing,
but that actually reading from it fails with EPERM.  The test assumes that if
FUSE is supported (/dev/fuse exists), then the testsuite is running on a kernel
with FUSE >= 7.27; I am unsure how to go about writing a test that validates
that the version check disables FUSE on old kernels.

[1]: https://github.com/systemd/systemd/issues/17607#issuecomment-745418835

Closes #17607

src/nspawn/nspawn-register.c

@@ -15,6 +15,7 @@
 
 static int append_machine_properties(
                 sd_bus_message *m,
+                bool enable_fuse,
                 CustomMount *mounts,
                 unsigned n_mounts,
                 int kill_signal,
@@ -40,6 +41,12 @@ static int append_machine_properties(
                                   "char-pts", "rw");
         if (r < 0)
                 return bus_log_create_error(r);
+        if (enable_fuse) {
+                r = sd_bus_message_append(m, "(sv)", "DeviceAllow", "a(ss)", 1,
+                                          "/dev/fuse", "rw");
+                if (r < 0)
+                        return bus_log_create_error(r);
+        }
 
         for (j = 0; j < n_mounts; j++) {
                 CustomMount *cm = mounts + j;
@@ -200,6 +207,7 @@ int register_machine(
 
                 r = append_machine_properties(
                                 m,
+                                FLAGS_SET(flags, REGISTER_MACHINE_ENABLE_FUSE),
                                 mounts,
                                 n_mounts,
                                 kill_signal,
@@ -320,6 +328,7 @@ int allocate_scope(
 
         r = append_machine_properties(
                         m,
+                        FLAGS_SET(flags, ALLOCATE_SCOPE_ENABLE_FUSE),
                         mounts,
                         n_mounts,
                         kill_signal,

src/nspawn/nspawn-register.h

@@ -9,7 +9,8 @@
 #include "nspawn-settings.h"
 
 typedef enum RegisterMachineFlags {
-        REGISTER_MACHINE_KEEP_UNIT = 1 << 0,
+        REGISTER_MACHINE_KEEP_UNIT   = 1 << 0,
+        REGISTER_MACHINE_ENABLE_FUSE = 1 << 1,
 } RegisterMachineFlags;
 
 int register_machine(
@@ -31,6 +32,7 @@ int unregister_machine(sd_bus *bus, const char *machine_name);
 
 typedef enum AllocateScopeFlags {
         ALLOCATE_SCOPE_ALLOW_PIDFD = 1 << 0,
+        ALLOCATE_SCOPE_ENABLE_FUSE = 1 << 1,
 } AllocateScopeFlags;
 
 int allocate_scope(

src/nspawn/nspawn.c

@@ -2,6 +2,7 @@
 
 #include <errno.h>
 #include <getopt.h>
+#include <linux/fuse.h>
 #include <linux/loop.h>
 #if HAVE_SELINUX
 #include <selinux/selinux.h>
@@ -2147,7 +2148,85 @@ static int setup_boot_id(void) {
         return mount_nofollow_verbose(LOG_ERR, NULL, to, NULL, MS_BIND|MS_REMOUNT|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL);
 }
 
-static int copy_devnodes(const char *dest) {
+static int get_fuse_version(uint32_t *ret_major, uint32_t *ret_minor) {
+        /* Must be called with mount privileges, either via arg_privileged or by being uid=0 in new
+         * CLONE_NEWUSER/CLONE_NEWNS namespaces. This is true when called from outer_child(). */
+        ssize_t n;
+        _cleanup_close_ int fuse_fd = -EBADF, mnt_fd = -EBADF;
+        _cleanup_free_ char *opts = NULL;
+        union {
+                char unstructured[FUSE_MIN_READ_BUFFER];
+                struct {
+                        struct fuse_in_header header;
+                        /* Don't use <linux/fuse.h>:`struct fuse_init_in` because a newer fuse.h might give
+                         * us a bigger struct than what an older kernel actually gives us, and that would
+                         * break our .header.len check. */
+                        struct {
+                                uint32_t major;
+                                uint32_t minor;
+                        } body;
+                } structured;
+        } request;
+
+        assert(ret_major);
+        assert(ret_minor);
+
+        /* Get a FUSE handle. */
+        fuse_fd = open("/dev/fuse", O_CLOEXEC|O_RDWR);
+        if (fuse_fd < 0)
+                return log_debug_errno(errno, "Failed to open /dev/fuse: %m");
+        if (asprintf(&opts, "fd=%i,rootmode=40000,user_id=0,group_id=0", fuse_fd) < 0)
+                return log_oom_debug();
+        mnt_fd = make_fsmount(LOG_DEBUG, "nspawn-fuse", "fuse.nspawn", 0, opts, -EBADF);
+        if (mnt_fd < 0)
+                return mnt_fd;
+
+        /* Read a request from the FUSE handle. */
+        n = read(fuse_fd, &request.unstructured, sizeof request);
+        if (n < 0)
+                return log_debug_errno(errno, "Failed to read /dev/fuse: %m");
+        if ((size_t) n < sizeof request.structured.header ||
+            (size_t) n < request.structured.header.len)
+                return log_debug_errno(SYNTHETIC_ERRNO(EIO), "Failed to read /dev/fuse: Short read");
+
+        /* Assume that the request is a FUSE_INIT request, and return the version information from it. */
+        if (request.structured.header.opcode != FUSE_INIT)
+                return log_debug_errno(SYNTHETIC_ERRNO(EIO), "Initial request from /dev/fuse should have opcode=%i (FUSE_INIT), but has opcode=%"PRIu32,
+                                       FUSE_INIT, request.structured.header.opcode);
+        if (request.structured.header.len < sizeof request.structured)
+                return log_debug_errno(SYNTHETIC_ERRNO(EIO), "Initial FUSE_INIT request from /dev/fuse is too short");
+        *ret_major = request.structured.body.major;
+        *ret_minor = request.structured.body.minor;
+        return 0;
+}
+
+static bool should_enable_fuse(void) {
+        uint32_t fuse_major, fuse_minor;
+        int r;
+
+        r = get_fuse_version(&fuse_major, &fuse_minor);
+        if (r < 0) {
+                if (ERRNO_IS_NEG_DEVICE_ABSENT(r))
+                        log_debug_errno(r, "Disabling FUSE: FUSE appears to be disabled on the host: %m");
+                else if (r == -ENOSYS)
+                        log_debug_errno(r, "Disabling FUSE: Kernel does not support the fsopen() family of syscalls: %m");
+                else
+                        log_warning_errno(r, "Disabling FUSE: Failed to determine FUSE version: %m");
+                return false;
+        }
+
+        /* FUSE is only userns-safe in FUSE version 7.27 and later.
+         * https://github.com/torvalds/linux/commit/da315f6e03988a7127680bbc26e1028991b899b8 */
+        if (fuse_major < 7 || (fuse_major == 7 && fuse_minor < 27)) {
+                log_debug("Disabling FUSE: FUSE version %" PRIu32 ".%" PRIu32 " is too old to support user namespaces",
+                          fuse_major, fuse_minor);
+                return false;
+        }
+
+        return true;
+}
+
+static int copy_devnodes(const char *dest, bool enable_fuse) {
         _cleanup_strv_free_ char **devnodes = NULL;
         int r = 0;
 
@@ -2159,6 +2238,7 @@ static int copy_devnodes(const char *dest) {
                             "random",
                             "urandom",
                             "tty",
+                            STRV_IFNOTNULL(enable_fuse ? "fuse" : NULL),
                             "net/tun");
         if (!devnodes)
                 return log_oom();
@@ -3807,7 +3887,7 @@ static int outer_child(
         _cleanup_(bind_user_context_freep) BindUserContext *bind_user_context = NULL;
         _cleanup_strv_free_ char **os_release_pairs = NULL;
         _cleanup_close_ int fd = -EBADF, mntns_fd = -EBADF;
-        bool idmap = false;
+        bool idmap = false, enable_fuse;
         const char *p;
         pid_t pid;
         ssize_t l;
@@ -4090,7 +4170,12 @@ static int outer_child(
         if (r < 0)
                 return r;
 
-        r = copy_devnodes(directory);
+        enable_fuse = should_enable_fuse();
+        l = send(fd_outer_socket, &enable_fuse, sizeof enable_fuse, 0);
+        if (l < 0)
+                return log_error_errno(errno, "Failed to send whether to enable FUSE: %m");
+
+        r = copy_devnodes(directory, enable_fuse);
         if (r < 0)
                 return r;
 
@@ -5048,6 +5133,7 @@ static int run_container(
         ssize_t l;
         sigset_t mask_chld;
         _cleanup_close_ int child_netns_fd = -EBADF;
+        bool enable_fuse;
 
         assert_se(sigemptyset(&mask_chld) == 0);
         assert_se(sigaddset(&mask_chld, SIGCHLD) == 0);
@@ -5234,6 +5320,12 @@ static int run_container(
                                                l, l == 0 ? " The child is most likely dead." : "");
         }
 
+        l = recv(fd_outer_socket_pair[0], &enable_fuse, sizeof enable_fuse, 0);
+        if (l < 0)
+                return log_error_errno(errno, "Failed to read whether to enable FUSE: %m");
+        if (l != sizeof enable_fuse)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Short read while reading whether to enable FUSE.");
+
         /* Wait for the outer child. */
         r = wait_for_terminate_and_check("(sd-namespace)", *pid, WAIT_LOG_ABNORMAL);
         if (r < 0)
@@ -5386,6 +5478,7 @@ static int run_container(
         if (arg_register) {
                 RegisterMachineFlags flags = 0;
                 SET_FLAG(flags, REGISTER_MACHINE_KEEP_UNIT, arg_keep_unit);
+                SET_FLAG(flags, REGISTER_MACHINE_ENABLE_FUSE, enable_fuse);
                 r = register_machine(
                                 bus,
                                 arg_machine,
@@ -5406,6 +5499,7 @@ static int run_container(
 
         } else if (!arg_keep_unit) {
                 AllocateScopeFlags flags = ALLOCATE_SCOPE_ALLOW_PIDFD;
+                SET_FLAG(flags, ALLOCATE_SCOPE_ENABLE_FUSE, enable_fuse);
                 r = allocate_scope(
                                 bus,
                                 arg_machine,

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -1112,4 +1112,75 @@ testcase_unpriv() {
     echo hello | cmp "$tmpdir/stdout.txt" -
 }
 
+testcase_fuse() {
+    if [[ "$(cat <>/dev/fuse 2>&1)" != 'cat: -: Operation not permitted' ]]; then
+        echo "FUSE is not supported, skipping the test..."
+        return 0
+    fi
+
+    # Assume that the tests are running on a kernel that is new enough for FUSE
+    # to have user-namespace support; and so we should expect that nspawn
+    # enables FUSE.  This test does not validate that the version check
+    # disables FUSE on old kernels.
+
+    local root
+
+    root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.fuse.XXX)"
+    create_dummy_container "$root"
+
+    # To avoid adding any complex dependencies to the test, we simply check
+    # that /dev/fuse can be opened for reading and writing (O_RDWR), but that
+    # actually reading from it fails with EPERM.  This can be done with a
+    # simple Bash script: run `cat <>/dev/fuse` and if the EPERM error message
+    # comes from "bash" then we know it couldn't be opened, while if it comes
+    # from "cat" then we know that it was opened but not read.  If we are able
+    # to read from the file, then this indicates that it's not a real FUSE
+    # device (which requires us to mount a type="fuse" filesystem with the
+    # option string "fd=${num}" for /dev/fuse FD before reading from it will
+    # return anything other than EPERM); if this happens then most likely
+    # nspawn didn't create the file at all and Bash "<>" simply created a new
+    # normal file.
+    #
+    #    "cat: -: Operation not permitted"                   # pass the test; opened but not read
+    #    "bash: line 1: /dev/fuse: Operation not permitted"  # fail the test; could not open
+    #    ""                                                  # fail the test; reading worked
+    [[ "$(systemd-nspawn --pipe --directory="$root" \
+              bash -c 'cat <>/dev/fuse' 2>&1)" == 'cat: -: Operation not permitted' ]]
+
+    rm -fr "$root"
+}
+
+testcase_unpriv_fuse() {
+    # Same as above, but for unprivileged operation.
+
+    if [[ "$(cat <>/dev/fuse 2>&1)" != 'cat: -: Operation not permitted' ]]; then
+        echo "FUSE is not supported, skipping the test..."
+        return 0
+    fi
+    if ! can_do_rootless_nspawn; then
+        echo "Skipping rootless test..."
+        return 0
+    fi
+
+    local tmpdir name
+    tmpdir="$(mktemp -d /var/tmp/TEST-13-NSPAWN.unpriv-fuse.XXX)"
+    # $name must be such that len("ns-$(id -u testuser)-nspawn-${name}-65535")
+    # <= 31, or nsresourced will reject the request for a namespace.
+    # Therefore; len($name) <= 10 bytes.
+    name="ufuse-${tmpdir##*.}"
+    trap 'rm -fr ${tmpdir@Q} || true; rm -f /run/verity.d/test-13-nspawn-${name@Q} || true' RETURN ERR
+    create_dummy_ddi "$tmpdir" "$name"
+    chown --recursive testuser: "$tmpdir"
+
+    [[ "$(systemd-run \
+              --pipe \
+              --uid=testuser \
+              --property=Delegate=yes \
+              --setenv=SYSTEMD_LOG_LEVEL \
+              --setenv=SYSTEMD_LOG_TARGET \
+              -- \
+              systemd-nspawn --pipe --private-network --register=no --keep-unit --image="$tmpdir/$name.raw" \
+                  bash -c 'cat <>/dev/fuse' 2>&1)" == *'cat: -: Operation not permitted' ]]
+}
+
 run_testcases

units/systemd-nspawn@.service.in

@@ -30,12 +30,16 @@ CoredumpReceive=yes
 TasksMax=16384
 {{SERVICE_WATCHDOG}}
 
-{# Enforce a strict device policy, similar to the one nspawn configures when it
- # allocates its own scope unit. Make sure to keep these policies in sync if you
- # change them! #}
+{# Enforce a strict device policy, similar to the one nspawn configures (in
+ # nspawn-register.c:append_machine_properties()) when it allocates its own
+ # scope unit. Make sure to keep these policies in sync if you change them! #}
 DevicePolicy=closed
 DeviceAllow=/dev/net/tun rwm
 DeviceAllow=char-pts rw
+{# /dev/fuse gets 'm' here even though it doesn't in nspawn-register.c, since
+ # efedb6b0f3 (nspawn: refuse to bind mount device node from host when
+ # --private-users= is specified, 2024-09-05) #}
+DeviceAllow=/dev/fuse rwm
 
 # nspawn itself needs access to /dev/loop-control and /dev/loop, to implement
 # the --image= option. Add these here, too.