scsi: ibmvfc: Fix OOB access in ibmvfc_discover_targets_done()

A malicious or compromised VIO server can return a num_written value in the
discover targets MAD response that exceeds max_targets. This value is
stored directly in vhost->num_targets without validation, and is then used
as the loop bound in ibmvfc_alloc_targets() to index into disc_buf[], which
is only allocated for max_targets entries. Indices at or beyond max_targets
access kernel memory outside the DMA-coherent allocation.  The
out-of-bounds data is subsequently embedded in Implicit Logout and PLOGI
MADs that are sent back to the VIO server, leaking kernel memory.

Fix by clamping num_written to max_targets before storing it.

Fixes: 072b91f ("[SCSI] ibmvfc: IBM Power Virtual Fibre Channel Adapter Client Driver")
Reported-by: Yuhao Jiang <[email protected]>
Cc: [email protected]
Signed-off-by: Tyllis Xu <[email protected]>
Reviewed-by: Dave Marquardt <[email protected]>
Acked-by: Tyrel Datwyler <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Martin K. Petersen <[email protected]>

Author: LivelyCarpet87

drivers/scsi/ibmvscsi/ibmvfc.c

@@ -4966,7 +4966,8 @@ static void ibmvfc_discover_targets_done(struct ibmvfc_event *evt)
 	switch (mad_status) {
 	case IBMVFC_MAD_SUCCESS:
 		ibmvfc_dbg(vhost, "Discover Targets succeeded\n");
-		vhost->num_targets = be32_to_cpu(rsp->num_written);
+		vhost->num_targets = min_t(u32, be32_to_cpu(rsp->num_written),
+					   max_targets);
 		ibmvfc_set_host_action(vhost, IBMVFC_HOST_ACTION_ALLOC_TGTS);
 		break;
 	case IBMVFC_MAD_FAILED: