From: David Lee Date: Wed, 1 Jul 2026 11:44:28 +0000 (+0000) Subject: fhandle: reject detached mounts in capable_wrt_mount() X-Git-Tag: v7.2-rc2~14^2~2 X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=6c732471740bc2ac9b0946134f9f551dc75f4369;p=thirdparty%2Flinux.git 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: stable@vger.kernel.org Signed-off-by: David Lee Assisted-by: LLM Link: https://patch.msgid.link/20260701114438.24431-1-david.lee@trailofbits.com Reviewed-by: Jeff Layton Signed-off-by: Christian Brauner (Amutable) --- 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,