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Wed, 01 Jul 2026 04:44:41 -0700 (PDT) From: David Lee To: viro@zeniv.linux.org.uk, Christian Brauner , Jan Kara Cc: David Lee , Chuck Lever , Jeff Layton , Amir Goldstein , Jann Horn , Dominik 'Disconnect3d' Czarnota , linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH] fhandle: reject detached mounts in capable_wrt_mount() Date: Wed, 1 Jul 2026 11:44:28 +0000 Message-ID: <20260701114438.24431-1-david.lee@trailofbits.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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: 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