SCSI: fix new bug in scsi_dev_info_list string matching

Commit b704f70ce200 ("SCSI: fix bug in scsi_dev_info_list matching")
changed the way vendor- and model-string matching was carried out in the
routine that looks up entries in a SCSI devinfo list. The new matching
code failed to take into account the case of a maximum-length string; in
such cases it could end up testing for a terminating '\0' byte beyond
the end of the memory allocated to the string. This out-of-bounds bug
was detected by UBSAN.

I don't know if anybody has actually encountered this bug. The symptom
would be that a device entry in the blacklist might not be matched
properly if it contained an 8-character vendor name or a 16-character
model name. Such entries certainly exist in scsi_static_device_list.

This patch fixes the problem by adding a check for a maximum-length
string before the '\0' test.

Signed-off-by: Alan Stern <[e-mail]>
Fixes: b704f70ce200 ("SCSI: fix bug in scsi_dev_info_list matching")
Tested-by: Wilfried Klaebe <[e-mail]>
CC: <[e-mail]> # v4.4+
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_devinfo.c

···
 	 * here, and we don't know what device it is
 	 * trying to work with, leave it as-is.
 	 */
-	vmax = 8;	/* max length of vendor */
+	vmax = sizeof(devinfo->vendor);
 	vskip = vendor;
 	while (vmax > 0 && *vskip == ' ') {
 		vmax--;
···
 	while (vmax > 0 && vskip[vmax - 1] == ' ')
 		--vmax;
 
-	mmax = 16;	/* max length of model */
+	mmax = sizeof(devinfo->model);
 	mskip = model;
 	while (mmax > 0 && *mskip == ' ') {
 		mmax--;
···
 			 * Behave like the older version of get_device_flags.
 			 */
 			if (memcmp(devinfo->vendor, vskip, vmax) ||
-					devinfo->vendor[vmax])
+					(vmax < sizeof(devinfo->vendor) &&
+						devinfo->vendor[vmax]))
 				continue;
 			if (memcmp(devinfo->model, mskip, mmax) ||
-					devinfo->model[mmax])
+					(mmax < sizeof(devinfo->model) &&
+						devinfo->model[mmax]))
 				continue;
 			return devinfo;
 		} else {