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    Subject[PATCH 5.12 113/700] fuse: Fix crash if superblock of submount gets killed early
    Date
    From: Greg Kurz <groug@kaod.org>

    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>

    ---
    fs/fuse/dir.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- a/fs/fuse/dir.c
    +++ b/fs/fuse/dir.c
    @@ -346,15 +346,15 @@ static struct vfsmount *fuse_dentry_auto
    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)) {

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