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 <jannh@google.com>
---
I used custom tooling to force this race condition to occur and check
that it leads to a KASAN splat - let me know if you want me to create a
kernel patch to force the race condition and a reproducer you can run.

I remember Christian asking me for feedback on the patch that introduced
the bug, and I missed the bug because I didn't realize what the semantics
of mount::mnt_ns are...
---
Changes in v2:
- improve comment on mnt_ns semantics based on discussion with viro@
- Link to v1: https://patch.msgid.link/20260603-vfs-fhandle-uaf-fix-v1-1-ff64ee367e4d@google.com
---
 fs/fhandle.c   | 16 ++++++++++++++--
 fs/mount.h     | 10 +++++++++-
 fs/namespace.c |  6 +++---
 3 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 642e3d569497..1ca7eb3a6cb5 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -285,6 +285,19 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
 	return 0;
 }
 
+static bool capable_wrt_mount(struct mount *mount)
+{
+	struct mnt_namespace *mnt_ns;
+
+	/*
+	 * For ->mnt_ns access.
+	 * The following READ_ONCE() is semantically rcu_dereference().
+	 */
+	guard(rcu)();
+	mnt_ns = READ_ONCE(mount->mnt_ns);
+	return ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN);
+}
+
 static inline int may_decode_fh(struct handle_to_path_ctx *ctx,
 				unsigned int o_flags)
 {
@@ -320,8 +333,7 @@ static inline int may_decode_fh(struct handle_to_path_ctx *ctx,
 	if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
 		ctx->flags = HANDLE_CHECK_PERMS;
 	else if (is_mounted(root->mnt) &&
-		 ns_capable(real_mount(root->mnt)->mnt_ns->user_ns,
-			    CAP_SYS_ADMIN) &&
+		 capable_wrt_mount(real_mount(root->mnt)) &&
 		 !has_locked_children(real_mount(root->mnt), root->dentry))
 		ctx->flags = HANDLE_CHECK_PERMS | HANDLE_CHECK_SUBTREE;
 	else
diff --git a/fs/mount.h b/fs/mount.h
index e0816c11a198..5c120f8361bd 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -71,7 +71,15 @@ struct mount {
 	struct hlist_head mnt_slave_list;/* list of slave mounts */
 	struct hlist_node mnt_slave;	/* slave list entry */
 	struct mount *mnt_master;	/* slave is on master->mnt_slave_list */
-	struct mnt_namespace *mnt_ns;	/* containing namespace */
+	/*
+	 * Containing namespace (active or deactivating, non-refcounted).
+	 * Normally protected by namespace_sem.
+	 * Can also be accessed locklessly under RCU. RCU readers can't rely on
+	 * the namespace still being active, but implicitly hold a passive
+	 * reference (because an RCU delay happens between a namespace being
+	 * deactivated and the corresponding passive refcount drop).
+	 */
+	struct mnt_namespace *mnt_ns;
 	struct mountpoint *mnt_mp;	/* where is it mounted */
 	union {
 		struct hlist_node mnt_mp_list;	/* list mounts with the same mountpoint */
diff --git a/fs/namespace.c b/fs/namespace.c
index fe919abd2f01..f5905f4ec560 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1079,7 +1079,7 @@ static void mnt_add_to_ns(struct mnt_namespace *ns, struct mount *mnt)
 	bool mnt_first_node = true, mnt_last_node = true;
 
 	WARN_ON(mnt_ns_attached(mnt));
-	mnt->mnt_ns = ns;
+	WRITE_ONCE(mnt->mnt_ns, ns);
 	while (*link) {
 		parent = *link;
 		if (mnt->mnt_id_unique < node_to_mount(parent)->mnt_id_unique) {
@@ -1434,7 +1434,7 @@ EXPORT_SYMBOL(mntget);
 void mnt_make_shortterm(struct vfsmount *mnt)
 {
 	if (mnt)
-		real_mount(mnt)->mnt_ns = NULL;
+		WRITE_ONCE(real_mount(mnt)->mnt_ns, NULL);
 }
 
 /**
@@ -1806,7 +1806,7 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 			ns->nr_mounts--;
 			__touch_mnt_namespace(ns);
 		}
-		p->mnt_ns = NULL;
+		WRITE_ONCE(p->mnt_ns, NULL);
 		if (how & UMOUNT_SYNC)
 			p->mnt.mnt_flags |= MNT_SYNC_UMOUNT;
 

---
base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
change-id: 20260603-vfs-fhandle-uaf-fix-32279d5b2758

Best regards,
--  
Jann Horn <jannh@google.com>

On Wed, 03 Jun 2026 21:31:57 +0200, Jann Horn wrote:
> 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):
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh()
      https://git.kernel.org/vfs/vfs/c/40ab6644b996