firewire: fw-sbp2: add a boundary check

Add rudimentary check for the case that the page table overflows due to
merging of s/g elements by the IOMMU.  This would have lead to
overwriting of arbitrary memory.

After this change I expect that an offending command will be
unsuccessfully retried until the scsi_device is taken offline by SCSI
core.  It's a border case and not worth to implement a recovery
strategy.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Kristian Høgsberg <krh@redhat.com>

drivers/firewire/fw-sbp2.c

@@ -937,6 +937,11 @@ static int sbp2_command_orb_map_scatterlist(struct sbp2_command_orb *orb)
 		sg_len = sg_dma_len(sg + i);
 		sg_addr = sg_dma_address(sg + i);
 		while (sg_len) {
+			/* FIXME: This won't get us out of the pinch. */
+			if (unlikely(j >= ARRAY_SIZE(orb->page_table))) {
+				fw_error("page table overflow\n");
+				goto fail_page_table;
+			}
 			l = min(sg_len, SBP2_MAX_SG_ELEMENT_LENGTH);
 			orb->page_table[j].low = sg_addr;
 			orb->page_table[j].high = (l << 16);