ACK: [PATCH][X] UBUNTU: SAUCE: (no-up) virtio-scsi: Fix race in target free

[Stefan Bader]

On 19.04.2018 21:40, Jay Vosburgh wrote:
> BugLink: http://bugs.launchpad.net/bugs/1765241
> 
> 	A race condition exists in virtio_scsi between the completion of
> a request and the freeing of the target structure.  The race is between
> (a) virtscsi_complete_cmd that, first, wakes up a task waiting for a
> completion, then, second, releases a reference in the target structure
> and (b) the woken up task freeing that target structure.
> 
> 	The race appears to exist in all verisons of virtio_scsi, but
> most kernels are not impacted due to a coincidental RCU sync in the
> "(b)" path above that will effectively wait for the "(a)" path to
> complete.  The Ubuntu Xenial 4.4 kernel since commit be2a20802abbde
> lacks any RCU sync in the "(b)" code path, thus opening the race window.
> 
> 	The fix is to wait for any outstanding requests to release their
> references prior to freeing the target structure.
> 
> Signed-off-by: Jay Vosburgh <jay.vosburgh at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>

> 
> ---

Sounds reasonable and safe enough.

-Stefan

>  drivers/scsi/virtio_scsi.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 8ef905cbfc9c..e2da31286793 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -785,6 +785,10 @@ static int virtscsi_target_alloc(struct scsi_target *starget)
>  static void virtscsi_target_destroy(struct scsi_target *starget)
>  {
>  	struct virtio_scsi_target_state *tgt = starget->hostdata;
> +
> +	/* we can race with concurrent virtscsi_complete_cmd */
> +	while (atomic_read(&tgt->reqs))
> +		cpu_relax();
>  	kfree(tgt);
>  }
>  
> 


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