kernel/linux.git/fs/namespace.c, branch v6.18.36

fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh()

2026-06-19T11:44:07+00:00

commit 40ab6644b99685755f740b872c00ef40d9aa870e upstream. may_decode_fh() accesses mount::mnt_ns without holding any locks; that means the mount can concurrently be unmounted, and the mnt_namespace can concurrently be freed after an RCU grace period. This race can happens as follows, assuming that the mount point was created by open_tree(..., OPEN_TREE_CLONE): thread 1 thread 2 RCU __do_sys_open_by_handle_at do_handle_open handle_to_path may_decode_fh is_mounted [mount::mnt_ns access] [mount::mnt_ns access] __do_sys_close fput_close_sync __fput dissolve_on_fput umount_tree class_namespace_excl_destructor namespace_unlock free_mnt_ns mnt_ns_tree_remove call_rcu(mnt_ns_release_rcu) mnt_ns_release_rcu mnt_ns_release kfree [mnt_namespace::user_ns access] **UAF** Fix it by taking rcu_read_lock() around the mount::mnt_ns access, like in __prepend_path(). Additionally, document the semantics of mount::mnt_ns, and use WRITE_ONCE() for writers that can race with lockless readers. This bug is unreachable unless one of the following is set: - CONFIG_PREEMPTION - CONFIG_RCU_STRICT_GRACE_PERIOD because it requires an RCU grace period to happen during a syscall without an explicit preemption. This doesn't seem to have interesting security impact; worst-case, it could leak the result of an integer comparison to userspace (from the level check in cap_capable()), cause an endless loop, or crash the kernel by dereferencing an invalid address. Fixes: 620c266f3949 ("fhandle: relax open_by_handle_at() permission checks") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn Link: https://patch.msgid.link/20260603-vfs-fhandle-uaf-fix-v2-1-d05db76a5084@google.com Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman

namespace: fix proc mount iteration

2026-03-12T11:09:34+00:00

commit 4a403d7aa9074f527f064ef0806aaab38d14b07c upstream. The m->index isn't updated when m->show() overflows and retains its value before the current mount causing a restart to start at the same value. If that happens in short order to due a quickly expanding mount table this would cause the same mount to be shown again and again. Ensure that *pos always equals the mount id of the mount that was returned by start/next. On restart after overflow mnt_find_id_at(*pos) finds the exact mount. This should avoid duplicates, avoid skips and should handle concurrent modification just fine. Cc: Fixed: 2eea9ce4310d8 ("mounts: keep list of mounts in an rbtree") Link: https://patch.msgid.link/20260129-geleckt-treuhand-4bb940acacd9@brauner Signed-off-by: Christian Brauner Signed-off-by: Greg Kroah-Hartman

fs: ensure that internal tmpfs mount gets mount id zero

2026-03-04T12:21:00+00:00

[ Upstream commit a2062463e894039a6fdc2334b96afd91d44b64a8 ] and the rootfs get mount id one as it always has. Before we actually mount the rootfs we create an internal tmpfs mount which has mount id zero but is never exposed anywhere. Continue that "tradition". Link: https://patch.msgid.link/20260112-work-immutable-rootfs-v2-1-88dd1c34a204@kernel.org Fixes: 7f9bfafc5f49 ("fs: use xarray for old mount id") Reviewed-by: Jeff Layton Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin

statmount: permission check should return EPERM

2026-03-04T12:19:28+00:00

[ Upstream commit fccbe38a5d06dbe44bcd89196fe1d2c2272a1f4a ] Currently, statmount() returns ENOENT when caller is not CAP_SYS_ADMIN in the user namespace owner of target mount namespace. This should be EPERM instead. Suggested-by: Miklos Szeredi Signed-off-by: Bhavik Sachdev Link: https://patch.msgid.link/20251129091455.757724-2-b.sachdev1904@gmail.com Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin

fs/namespace: fix reference leak in grab_requested_mnt_ns

2025-11-25T08:34:56+00:00

lookup_mnt_ns() already takes a reference on mnt_ns. grab_requested_mnt_ns() doesn't need to take an extra reference. Fixes: 78f0e33cd6c93 ("fs/namespace: correctly handle errors returned by grab_requested_mnt_ns") Signed-off-by: Andrei Vagin Link: https://patch.msgid.link/20251122071953.3053755-1-avagin@google.com Signed-off-by: Christian Brauner

fs/namespace: correctly handle errors returned by grab_requested_mnt_ns

2025-11-12T09:42:49+00:00

grab_requested_mnt_ns was changed to return error codes on failure, but its callers were not updated to check for error pointers, still checking only for a NULL return value. This commit updates the callers to use IS_ERR() or IS_ERR_OR_NULL() and PTR_ERR() to correctly check for and propagate errors. This also makes sure that the logic actually works and mount namespace file descriptors can be used to refere to mounts. Christian Brauner says: Rework the patch to be more ergonomic and in line with our overall error handling patterns. Fixes: 7b9d14af8777 ("fs: allow mount namespace fd") Cc: Christian Brauner Signed-off-by: Andrei Vagin Link: https://patch.msgid.link/20251111062815.2546189-1-avagin@google.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner

mnt: Remove dead code which might prevent from building

2025-10-29T14:57:24+00:00

Clang, in particular, is not happy about dead code: fs/namespace.c:135:37: error: unused function 'node_to_mnt_ns' [-Werror,-Wunused-function] 135 | static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node) | ^~~~~~~~~~~~~~ 1 error generated. Remove a leftover from the previous cleanup. Fixes: 7d7d16498958 ("mnt: support ns lookup") Signed-off-by: Andy Shevchenko Link: https://patch.msgid.link/20251024132336.1666382-1-andriy.shevchenko@linux.intel.com Signed-off-by: Christian Brauner

fs: Fix uninitialized 'offp' in statmount_string()

2025-10-21T12:21:46+00:00

In statmount_string(), most flags assign an output offset pointer (offp) which is later updated with the string offset. However, the STATMOUNT_MNT_UIDMAP and STATMOUNT_MNT_GIDMAP cases directly set the struct fields instead of using offp. This leaves offp uninitialized, leading to a possible uninitialized dereference when *offp is updated. Fix it by assigning offp for UIDMAP and GIDMAP as well, keeping the code path consistent. Fixes: 37c4a9590e1e ("statmount: allow to retrieve idmappings") Fixes: e52e97f09fb6 ("statmount: let unset strings be empty") Cc: stable@vger.kernel.org Signed-off-by: Zhen Ni Link: https://patch.msgid.link/20251013114151.664341-1-zhen.ni@easystack.cn Reviewed-by: Jan Kara Signed-off-by: Christian Brauner

Merge tag 'pull-fs_context' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

2025-10-03T17:51:44+00:00

Pull fs_context updates from Al Viro: "Change vfs_parse_fs_string() calling conventions Get rid of the length argument (almost all callers pass strlen() of the string argument there), add vfs_parse_fs_qstr() for the cases that do want separate length" * tag 'pull-fs_context' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: do_nfs4_mount(): switch to vfs_parse_fs_string() change the calling conventions for vfs_parse_fs_string()

Merge tag 'pull-mount' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

2025-10-03T17:19:44+00:00

Pull vfs mount updates from Al Viro: "Several piles this cycle, this mount-related one being the largest and trickiest: - saner handling of guards in fs/namespace.c, getting rid of needlessly strong locking in some of the users - lock_mount() calling conventions change - have it set the environment for attaching to given location, storing the results in caller-supplied object, without altering the passed struct path. Make unlock_mount() called as __cleanup for those objects. It's not exactly guard(), but similar to it - MNT_WRITE_HOLD done right. mnt_hold_writers() does *not* mess with ->mnt_flags anymore, so insertion of a new mount into ->s_mounts of underlying superblock does not, in itself, expose ->mnt_flags of that mount to concurrent modifications - getting rid of pathological cases when umount() spends quadratic time removing the victims from propagation graph - part of that had been dealt with last cycle, this should finish it - a bunch of stuff constified - assorted cleanups * tag 'pull-mount' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (64 commits) constify {__,}mnt_is_readonly() WRITE_HOLD machinery: no need for to bump mount_lock seqcount struct mount: relocate MNT_WRITE_HOLD bit preparations to taking MNT_WRITE_HOLD out of ->mnt_flags setup_mnt(): primitive for connecting a mount to filesystem simplify the callers of mnt_unhold_writers() copy_mnt_ns(): use guards copy_mnt_ns(): use the regular mechanism for freeing empty mnt_ns on failure open_detached_copy(): separate creation of namespace into helper open_detached_copy(): don't bother with mount_lock_hash() path_has_submounts(): use guard(mount_locked_reader) fs/namespace.c: sanitize descriptions for {__,}lookup_mnt() ecryptfs: get rid of pointless mount references in ecryptfs dentries umount_tree(): take all victims out of propagation graph at once do_mount(): use __free(path_put) do_move_mount_old(): use __free(path_put) constify can_move_mount_beneath() arguments path_umount(): constify struct path argument may_copy_tree(), __do_loopback(): constify struct path argument path_mount(): constify struct path argument ...