[PATCH v2 10/17] mpi3mr: Fix out-of-bounds read in SAS topology change events

[Chandrakanth Patil <chandrakanth.patil@broadcom.com>]

The number of entries in a SAS topology change event is used to walk
the entry array without being compared against the amount of event data
that was received, so the walk can run past the end of the buffer.

Work out how many entries the payload can hold and skip the event if it
claims more.

Fixes: 13ef29ea4aa0 ("scsi: mpi3mr: Add support for device add/remove event handling")
Signed-off-by: Chandrakanth Patil <chandrakanth.patil@broadcom.com>
---
v2:
 - Cached num_entries in a local variable before bounds check and used it
   for the loop condition to prevent re-fetching from DMA memory (TOCTOU).
 drivers/scsi/mpi3mr/mpi3mr_os.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/mpi3mr/mpi3mr_os.c b/drivers/scsi/mpi3mr/mpi3mr_os.c
index 9aeec6146acb..6a80b784200b 100644
--- a/drivers/scsi/mpi3mr/mpi3mr_os.c
+++ b/drivers/scsi/mpi3mr/mpi3mr_os.c
@@ -2827,12 +2827,28 @@ static void mpi3mr_sastopochg_evt_th(struct mpi3mr_ioc *mrioc,
 	struct mpi3_event_data_sas_topology_change_list *topo_evt =
 	    (struct mpi3_event_data_sas_topology_change_list *)event_reply->event_data;
 	int i;
-	u16 handle;
-	u8 reason_code;
+	u16 handle, avail_len;
+	u8 reason_code, max_entries, num_entries;
 	struct mpi3mr_tgt_dev *tgtdev = NULL;
 	struct mpi3mr_stgt_priv_data *scsi_tgt_priv_data = NULL;
 
-	for (i = 0; i < topo_evt->num_entries; i++) {
+	avail_len = event_reply->event_data_length * 4;
+	if (avail_len < offsetof(struct mpi3_event_data_sas_topology_change_list, phy_entry)) {
+		ioc_err(mrioc, "SAS topology event: event data too small (%u bytes)\n",
+			avail_len);
+		return;
+	}
+	max_entries = (avail_len -
+		      offsetof(struct mpi3_event_data_sas_topology_change_list, phy_entry)) /
+		      sizeof(struct mpi3_event_sas_topo_phy_entry);
+	num_entries = topo_evt->num_entries;
+	if (num_entries > max_entries) {
+		ioc_err(mrioc, "SAS topology event: num_entries(%d) exceeds max(%d)\n",
+			num_entries, max_entries);
+		return;
+	}
+
+	for (i = 0; i < num_entries; i++) {
 		handle = le16_to_cpu(topo_evt->phy_entry[i].attached_dev_handle);
 		if (!handle)
 			continue;
-- 
2.52.0