fuse: Fix crash if superblock of submount gets killed early

commit e3a43f2a95393000778f8f302d48795add2fc4a8 upstream.

As soon as fuse_dentry_automount() does up_write(&sb->s_umount), the
superblock can theoretically be killed. If this happens before the
submount was added to the &fc->mounts list, fuse_mount_remove() later
crashes in list_del_init() because it assumes the submount to be
already there.

Add the submount before dropping sb->s_umount to fix the inconsistency.
It is okay to nest fc->killsb under sb->s_umount, we already do this
on the ->kill_sb() path.

Signed-off-by: Greg Kurz <groug@kaod.org>
Fixes: bf109c64040f ("fuse: implement crossmounts")
Cc: stable@vger.kernel.org # v5.10+
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index d99bf6207b167f79a05089e54092b9cbf8368333..14958cedf4144d9bd2675cae2ade3c3be7034093 100644 (file)
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -347,15 +347,15 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
                goto out_put_sb;
        }
 
+       down_write(&fc->killsb);
+       list_add_tail(&fm->fc_entry, &fc->mounts);
+       up_write(&fc->killsb);
+
        sb->s_flags |= SB_ACTIVE;
        fsc->root = dget(sb->s_root);
        /* We are done configuring the superblock, so unlock it */
        up_write(&sb->s_umount);
 
-       down_write(&fc->killsb);
-       list_add_tail(&fm->fc_entry, &fc->mounts);
-       up_write(&fc->killsb);
-
        /* Create the submount */
        mnt = vfs_create_mount(fsc);
        if (IS_ERR(mnt)) {