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, 14 insertions, 2 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