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fuse: Fix crash if superblock of submount gets killed early

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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: bf109c6404 ("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>
This commit is contained in:
Greg Kurz 2021-06-04 18:11:51 +02:00 committed by Miklos Szeredi
parent d92d88f056
commit e3a43f2a95
1 changed files with 4 additions and 4 deletions
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8
fs/fuse/dir.c
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@ -346,15 +346,15 @@ 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)) {