Re: [PATCH v2] loop: call __loop_clr_fd() with lo_mutex locked to avoid autoclear race

[Pavel Tatashin]

On Fri, Mar 26, 2021 at 5:00 AM <qiang.zhang@xxxxxxxxxxxxx> wrote:
>
> From: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
>
> lo->lo_refcnt = 0
>
>             CPU0                                 CPU1
> lo_open()                                    lo_open()
>  mutex_lock(&lo->lo_mutex)
>  atomic_inc(&lo->lo_refcnt)
>  lo_refcnt == 1
>  mutex_unlock(&lo->lo_mutex)
>                                              mutex_lock(&lo->lo_mutex)
>                                              atomic_inc(&lo->lo_refcnt)
>                                              lo_refcnt == 2
>                                              mutex_unlock(&lo->lo_mutex)
> loop_clr_fd()
>  mutex_lock(&lo->lo_mutex)
>  atomic_read(&lo->lo_refcnt) > 1
>  lo->lo_flags |= LO_FLAGS_AUTOCLEAR        lo_release()
>  mutex_unlock(&lo->lo_mutex)
>  return                                      mutex_lock(&lo->lo_mutex)
>                                            atomic_dec_return(&lo->lo_refcnt)
>                                              lo_refcnt == 1
>                                              mutex_unlock(&lo->lo_mutex)
>                                              return
>
> lo_release()
>  mutex_lock(&lo->lo_mutex)
>  atomic_dec_return(&lo->lo_refcnt)
>  lo_refcnt == 0
>  lo->lo_flags & LO_FLAGS_AUTOCLEAR
>   == true
>  mutex_unlock(&lo->lo_mutex)              loop_control_ioctl()
>                                            case LOOP_CTL_REMOVE:
>                                             mutex_lock(&lo->lo_mutex)
>                                             atomic_read(&lo->lo_refcnt)==0
>   __loop_clr_fd(lo, true)                   mutex_unlock(&lo->lo_mutex)
>     mutex_lock(&lo->lo_mutex)                loop_remove(lo)
>                                                mutex_destroy(&lo->lo_mutex)
>   ......                                       kfree(lo)
>        data race
>
> When different tasks on two CPUs perform the above operations on the same
> lo device, data race may be occur, Do not drop lo->lo_mutex before calling
>  __loop_clr_fd(), so refcnt and LO_FLAGS_AUTOCLEAR check in lo_release
> stay in sync.

There is a race with autoclear logic where use after free may occur as
shown in the above scenario. Do not drop lo->lo_mutex before calling
__loop_clr_fd(), so refcnt and LO_FLAGS_AUTOCLEAR check in lo_release
stay in sync.

Reviewed-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

>
> Fixes: 6cc8e7430801 ("loop: scale loop device by introducing per device lock")
> Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> ---
>  v1->v2:
>  Modify the title and commit message.
>
>  drivers/block/loop.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index d58d68f3c7cd..5712f1698a66 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1201,7 +1201,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>         bool partscan = false;
>         int lo_number;
>
> -       mutex_lock(&lo->lo_mutex);
>         if (WARN_ON_ONCE(lo->lo_state != Lo_rundown)) {
>                 err = -ENXIO;
>                 goto out_unlock;
> @@ -1257,7 +1256,6 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>         lo_number = lo->lo_number;
>         loop_unprepare_queue(lo);
>  out_unlock:
> -       mutex_unlock(&lo->lo_mutex);
>         if (partscan) {
>                 /*
>                  * bd_mutex has been held already in release path, so don't
> @@ -1288,12 +1286,11 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>          * protects us from all the other places trying to change the 'lo'
>          * device.
>          */
> -       mutex_lock(&lo->lo_mutex);
> +
>         lo->lo_flags = 0;
>         if (!part_shift)
>                 lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
>         lo->lo_state = Lo_unbound;
> -       mutex_unlock(&lo->lo_mutex);
>
>         /*
>          * Need not hold lo_mutex to fput backing file. Calling fput holding
> @@ -1332,9 +1329,10 @@ static int loop_clr_fd(struct loop_device *lo)
>                 return 0;
>         }
>         lo->lo_state = Lo_rundown;
> +       err = __loop_clr_fd(lo, false);
>         mutex_unlock(&lo->lo_mutex);
>
> -       return __loop_clr_fd(lo, false);
> +       return err;
>  }
>
>  static int
> @@ -1916,13 +1914,12 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
>                 if (lo->lo_state != Lo_bound)
>                         goto out_unlock;
>                 lo->lo_state = Lo_rundown;
> -               mutex_unlock(&lo->lo_mutex);
>                 /*
>                  * In autoclear mode, stop the loop thread
>                  * and remove configuration after last close.
>                  */
>                 __loop_clr_fd(lo, true);
> -               return;
> +               goto out_unlock;
>         } else if (lo->lo_state == Lo_bound) {
>                 /*
>                  * Otherwise keep thread (if running) and config,
> --
> 2.17.1
>