[CIFS] Check that last search entry resume key is valid

Jeff's recent patch to add a last_entry field in the search structure
to better construct resume keys did not validate that the server
sent us a plausible pointer to the last entry.  This adds that.
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifssmb.c

@@ -3614,6 +3614,8 @@ findFirstRetry:
 		/* BB remember to free buffer if error BB */
 		rc = validate_t2((struct smb_t2_rsp *)pSMBr);
 		if (rc == 0) {
+			unsigned int lnoff;
+
 			if (pSMBr->hdr.Flags2 & SMBFLG2_UNICODE)
 				psrch_inf->unicode = true;
 			else
@@ -3636,8 +3638,17 @@ findFirstRetry:
 					le16_to_cpu(parms->SearchCount);
 			psrch_inf->index_of_last_entry = 2 /* skip . and .. */ +
 				psrch_inf->entries_in_buffer;
+			lnoff = le16_to_cpu(parms->LastNameOffset);
+			if (tcon->ses->server->maxBuf - MAX_CIFS_HDR_SIZE <
+			      lnoff) {
+				cERROR(1, ("ignoring corrupt resume name"));
+				psrch_inf->last_entry = NULL;
+				return rc;
+			}
+
 			psrch_inf->last_entry = psrch_inf->srch_entries_start +
-					le16_to_cpu(parms->LastNameOffset);
+							lnoff;
+
 			*pnetfid = parms->SearchHandle;
 		} else {
 			cifs_buf_release(pSMB);
@@ -3727,6 +3738,8 @@ int CIFSFindNext(const int xid, struct cifsTconInfo *tcon,
 		rc = validate_t2((struct smb_t2_rsp *)pSMBr);
 
 		if (rc == 0) {
+			unsigned int lnoff;
+
 			/* BB fixme add lock for file (srch_info) struct here */
 			if (pSMBr->hdr.Flags2 & SMBFLG2_UNICODE)
 				psrch_inf->unicode = true;
@@ -3753,8 +3766,16 @@ int CIFSFindNext(const int xid, struct cifsTconInfo *tcon,
 						le16_to_cpu(parms->SearchCount);
 			psrch_inf->index_of_last_entry +=
 				psrch_inf->entries_in_buffer;
-			psrch_inf->last_entry = psrch_inf->srch_entries_start +
-					le16_to_cpu(parms->LastNameOffset);
+			lnoff = le16_to_cpu(parms->LastNameOffset);
+			if (tcon->ses->server->maxBuf - MAX_CIFS_HDR_SIZE <
+			      lnoff) {
+				cERROR(1, ("ignoring corrupt resume name"));
+				psrch_inf->last_entry = NULL;
+				return rc;
+			} else
+				psrch_inf->last_entry =
+					psrch_inf->srch_entries_start + lnoff;
+
 /*  cFYI(1,("fnxt2 entries in buf %d index_of_last %d",
 	    psrch_inf->entries_in_buffer, psrch_inf->index_of_last_entry)); */