[PATCH] scsi: ibmvfc: fix OOB access in ibmvfc_discover_targets_done()

[Tyllis Xu]

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: 072b91f9c651 ("[SCSI] ibmvfc: IBM Power Virtual Fibre Channel Adapter Client Driver")
Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tyllis Xu <LivelyCarpet87@xxxxxxxxx>
---
 drivers/scsi/ibmvscsi/ibmvfc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/ibmvscsi/ibmvfc.c b/drivers/scsi/ibmvscsi/ibmvfc.c
index a20fce04fe79..3dd2adda195e 100644
--- a/drivers/scsi/ibmvscsi/ibmvfc.c
+++ b/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:
-- 
2.43.0