[PATCH] virtio-blk: fix short scsi inhdr host OOB write

[Jia Jia <physicalmtea@gmail.com>]

virtio_blk_handle_scsi() only validates the input/output descriptor
counts and then unconditionally treats the second-to-last input
descriptor as a struct virtio_scsi_inhdr.

If that descriptor is shorter than struct virtio_scsi_inhdr, the host
still performs a 4-byte virtio_stl_p() store while writing scsi->errors.

This is reproducible as a host-side heap-buffer-overflow under ASAN:

  ==4022698==ERROR: AddressSanitizer: heap-buffer-overflow on address
  0x504000023570 at pc 0x5e4be9c09800 bp 0x7ffebf4d7510 sp 0x7ffebf4d7500
  WRITE of size 4 at 0x504000023570 thread T0
      #0 0x5e4be9c097ff in stl_he_p include/qemu/bswap.h:284
      #1 0x5e4be9c09c4d in stl_le_p include/qemu/bswap.h:331
      #2 0x5e4be9c0a48b in virtio_stl_p include/hw/virtio/virtio-access.h:38
      #3 0x5e4be9c0c201 in virtio_blk_handle_scsi ../hw/block/virtio-blk.c:207
      #4 0x5e4be9c1578b in virtio_blk_handle_request ../hw/block/virtio-blk.c:926
      #5 0x5e4be9c160e3 in virtio_blk_handle_vq ../hw/block/virtio-blk.c:1025
      #6 0x5e4be9c16529 in virtio_blk_handle_output ../hw/block/virtio-blk.c:1058
      #7 0x5e4bea713ad9 in virtio_queue_notify_vq ../hw/virtio/virtio.c:2507
      #8 0x5e4bea724bfc in virtio_queue_host_notifier_read ../hw/virtio/virtio.c:3981

The same run shows the short descriptor being mapped through the
bounce-buffer path:

  allocated by thread T0 here:
      #0 0x736faf8b4a57 in __interceptor_calloc
      #1 0x736faf1a5c50 in g_malloc0
      #2 0x5e4bea925458 in address_space_map ../system/physmem.c:3746
      #3 0x5e4bea6f7633 in dma_memory_map include/system/dma.h:212
      #4 0x5e4bea70610a in virtqueue_map_desc ../hw/virtio/virtio.c:1637
      #5 0x5e4bea70824e in virtqueue_split_pop ../hw/virtio/virtio.c:1817
      #6 0x5e4bea70c9a8 in virtqueue_pop ../hw/virtio/virtio.c:2039
      #7 0x5e4be9c0be03 in virtio_blk_get_request ../hw/block/virtio-blk.c:172

Reject requests whose second-to-last input descriptor is too short to
hold struct virtio_scsi_inhdr.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
 hw/block/virtio-blk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 9cb9f1fb2b..418e0dd9c6 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -197,6 +197,12 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
         goto fail;
     }
 
+    if (elem->in_sg[elem->in_num - 2].iov_len <
+        sizeof(struct virtio_scsi_inhdr)) {
+        status = VIRTIO_BLK_S_IOERR;
+        goto fail;
+    }
+
     /*
      * The scsi inhdr is placed in the second-to-last input segment, just
      * before the regular inhdr.
-- 
2.34.1