[PATCH] vhost-scsi: fix event queue iov out-of-bounds

[Jia Jia <physicalmtea@gmail.com>]

vhost_scsi_do_evt_work() uses vq->iov[out] after vhost_get_vq_desc()
without checking that the descriptor chain contains an input segment.
vq->iov has UIO_MAXIOV entries (1024).  A chain of 1024 output-only
descriptors yields out == 1024 and in == 0, so the length check reads
one past the end of the array.

With a host-side harness that sets VHOST_SCSI_SET_EVENTS_MISSED and
kicks an event queue filled with 1024 OUT descriptors, UBSAN reports:

  UBSAN: array-index-out-of-bounds in drivers/vhost/scsi.c:611:14
  index 1024 is out of range for type 'iovec [1024]'

  Call Trace:
   <TASK>
   dump_stack_lvl+0x5f/0x90
   dump_stack+0x10/0x18
   ubsan_epilogue+0x9/0x39
   __ubsan_handle_out_of_bounds.cold+0x50/0x55
   vhost_scsi_complete_events+0x55f/0x5a0 [vhost_scsi]
   vhost_scsi_evt_work+0x17/0x30 [vhost_scsi]
   vhost_run_work_list+0x8e/0xd0 [vhost]
   vhost_task_fn+0xe1/0x210
   ret_from_fork+0x348/0x540
   ret_from_fork_asm+0x1a/0x30
   </TASK>

Require at least one input descriptor before indexing iov[out], and
treat a pure-output chain as a missed event like other invalid event
buffers.

Fixes: a6c9af87363c ("tcm_vhost: Add hotplug/hotunplug support")
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
 drivers/vhost/scsi.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 9a1253b9d8c5..9b5f07f52b0c 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -608,9 +608,10 @@
 		return;
 	}
 
-	if ((vq->iov[out].iov_len != sizeof(struct virtio_scsi_event))) {
+	if (!in ||
+	    vq->iov[out].iov_len != sizeof(struct virtio_scsi_event)) {
 		vq_err(vq, "Expecting virtio_scsi_event, got %zu bytes\n",
-				vq->iov[out].iov_len);
+		       in ? vq->iov[out].iov_len : 0);
 		vs->vs_events_missed = true;
 		return;
 	}
-- 
2.43.0