Re: [PATCH v2] vhost-scsi: do not count PI bytes in the data length

[Michael S. Tsirkin]

On Sun, Oct 11, 2026 at 09:48:03AM +0800, Jia Jia wrote:
> vhost_scsi_handle_vq() moves pi_bytesout or pi_bytesin into the
> protection sgl and subtracts those bytes from exp_data_len.  The length
> passed to TCM is still exp_data_len + prot_bytes.
> 
> prot_bytes comes from virtio_scsi_cmd_req_pi.  With
> VIRTIO_SCSI_F_T10_PI negotiated, vhost reads that header and does not
> check that the CDB is a READ or WRITE.  sbc_check_prot() replaces
> data_length from the CDB only when the protect field is non-zero.
> target_cmd_size_check() leaves a write's data_length unchanged when the
> CDB size is smaller, so a command that does not get that replacement
> keeps a data_length larger than the data sgl.
> 
> The KASAN report is a MODE SELECT whose header had pi_bytesout set.
> The data sgl held only the parameter list.  spc_emulate_modeselect()
> walked data_length and read the next physical page.
> 
> Pass the data sgl length.  Protected READ and WRITE still take their
> length from the CDB in sbc_check_prot().
> 
> KASAN reports:
> 
>   BUG: KASAN: use-after-free in spc_emulate_modeselect+0x145/0x380 [target_core_mod]
>   Read of size 1
> 
>   spc_emulate_modeselect
>   __target_execute_cmd
>   target_execute_cmd
>   vhost_scsi_write_pending
>   transport_generic_new_cmd
>   __target_submit
>   target_queued_submit_work
>   process_one_work
>   worker_thread
>   kthread
>   ret_from_fork
>   ret_from_fork_asm
> 
> Fixes: 9f977ef7b671 ("vhost-scsi: Include prot_bytes into expected data transfer length")
> Signed-off-by: Jia Jia <physicalmtea@xxxxxxxxx>
> ---
> v2:
>   - Drop the claim that target used to subtract prot_length.
>     9f977ef7b671 cites 14ef9200, which is not in the tree
>   - prot_bytes comes from virtio_scsi_cmd_req_pi


you can mention the other patches you sent fixing
security maybe this will calm down sashiko.

> 
>  drivers/vhost/scsi.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 4f8c0260bc9e..4bd29c2ceed9 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -1534,9 +1534,13 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
>  		 * vhost_scsi_queue_data_in() and vhost_scsi_queue_status()
>  		 */
>  		cmd->tvc_vq_desc = vc.head;
> +		/*
> +		 * exp_data_len is the data sgl. prot_bytes already sit in
> +		 * the protection sgl. Protected READ and WRITE replace
> +		 * data_length from the CDB in sbc_check_prot().

this is barely readable - could you improve pls? even if that makes it
a bit more verbose.




> +		 */
>  		vhost_scsi_target_queue_cmd(nexus, cmd, cdb, lun, task_attr,
> -					    data_direction,
> -					    exp_data_len + prot_bytes);
> +					    data_direction, exp_data_len);
>  		ret = 0;
>  err:
>  		/*
> -- 
> 2.53.0