vhost-scsi: Validate T10 PI scatterlist counts

When T10 PI is negotiated, vhost-scsi splits protection bytes from
the data iterator before mapping the request scatterlists. A malformed
request can claim protection bytes that cover or exceed the full payload
length. The former leaves no data bytes to map, while the latter
underflows exp_data_len before advancing the iterator. Both cases can let
a zero data SGL count reach sg_alloc_table_chained(), which triggers
BUG_ON(!nents).

Reject protection lengths that cover or exceed the payload before
subtracting prot_bytes and advancing the iterator. Also propagate
negative errors from the protection SGL calculation before calling the
allocator, matching the data SGL path.

Fixes: bca939d5bc ("vhost-scsi: Dynamically allocate scatterlists")
Suggested-by: Jia Jia <[email protected]>
Signed-off-by: Jia Jia <[email protected]>
Assisted-by: OpenAI-Codex:GPT-5
Signed-off-by: Linfeng Sun <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

drivers/vhost/scsi.c

@@ -972,6 +972,9 @@ vhost_scsi_mapal(struct vhost_scsi *vs, struct vhost_scsi_cmd *cmd,
 	if (prot_bytes) {
 		sgl_count = vhost_scsi_calc_sgls(prot_iter, prot_bytes,
 						 VHOST_SCSI_PREALLOC_PROT_SGLS);
+		if (sgl_count < 0)
+			return sgl_count;
+
 		cmd->prot_table.sgl = cmd->prot_sgl;
 		ret = sg_alloc_table_chained(&cmd->prot_table, sgl_count,
 					     cmd->prot_table.sgl,
@@ -1416,6 +1419,11 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 			 * actual data payload length.
 			 */
 			if (prot_bytes) {
+				if (prot_bytes >= exp_data_len) {
+					vq_err(vq, "Protection data exceeds payload length\n");
+					goto err;
+				}
+
 				exp_data_len -= prot_bytes;
 				prot_iter = data_iter;
 				iov_iter_truncate(&prot_iter, prot_bytes);