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

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>
Link: https://patch.msgid.link/20260603-vfs-fhandle-uaf-fix-v2-1-d05db76a5084@google.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 9 insertions, 1 deletions

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 */