[PATCH] fhandle: reject detached mounts in capable_wrt_mount()

The recent fhandle RCU fix moved the mount namespace capability check
into capable_wrt_mount(), so a non-NULL mnt_namespace survives the
ns_capable() dereference. The helper still assumes the later
READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh()
checked is_mounted() first.

That assumption is not stable. A detached mount from
open_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while
open_by_handle_at() is between those checks, and umount_tree() can
clear mount->mnt_ns. If the helper observes NULL, it dereferences
mnt_ns->user_ns and panics.

Return false when the RCU read observes a detached mount. This keeps
the relaxed permission path conservative: a mount no longer attached
to a namespace cannot authorize open_by_handle_at() access.

Fixes: 620c266f3949 ("fhandle: relax open_by_handle_at() permission checks")
Cc: [email protected]
Signed-off-by: David Lee <[email protected]>
Assisted-by: Codex:gpt-5
---
Bug found and triaged by David Lee from Trail of Bits.

Trail of Bits has a minimal PoC that triggers this crash on a custom
kernel build, which can be shared further if needed.

 fs/fhandle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 1ca7eb3a6cb5..f8829231e3d7 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -295,7 +295,7 @@ static bool capable_wrt_mount(struct mount *mount)
 	 */
 	guard(rcu)();
 	mnt_ns = READ_ONCE(mount->mnt_ns);
-	return ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN);
+	return mnt_ns && ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN);
 }
 
 static inline int may_decode_fh(struct handle_to_path_ctx *ctx,
-- 
2.43.0