CIFS: fix wrapping bugs in num_entries()

The problem is that "entryptr + next_offset" and "entryptr + len + size"
can wrap.  I ended up changing the type of "entryptr" because it makes
the math easier when we don't have to do so much casting.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2pdu.c

@@ -3577,33 +3577,38 @@ num_entries(char *bufstart, char *end_of_buf, char **lastentry, size_t size)
 	int len;
 	unsigned int entrycount = 0;
 	unsigned int next_offset = 0;
-	FILE_DIRECTORY_INFO *entryptr;
+	char *entryptr;
+	FILE_DIRECTORY_INFO *dir_info;
 
 	if (bufstart == NULL)
 		return 0;
 
-	entryptr = (FILE_DIRECTORY_INFO *)bufstart;
+	entryptr = bufstart;
 
 	while (1) {
-		entryptr = (FILE_DIRECTORY_INFO *)
-					((char *)entryptr + next_offset);
-
-		if ((char *)entryptr + size > end_of_buf) {
+		if (entryptr + next_offset < entryptr ||
+		    entryptr + next_offset > end_of_buf ||
+		    entryptr + next_offset + size > end_of_buf) {
 			cifs_dbg(VFS, "malformed search entry would overflow\n");
 			break;
 		}
 
-		len = le32_to_cpu(entryptr->FileNameLength);
-		if ((char *)entryptr + len + size > end_of_buf) {
+		entryptr = entryptr + next_offset;
+		dir_info = (FILE_DIRECTORY_INFO *)entryptr;
+
+		len = le32_to_cpu(dir_info->FileNameLength);
+		if (entryptr + len < entryptr ||
+		    entryptr + len > end_of_buf ||
+		    entryptr + len + size > end_of_buf) {
 			cifs_dbg(VFS, "directory entry name would overflow frame end of buf %p\n",
 				 end_of_buf);
 			break;
 		}
 
-		*lastentry = (char *)entryptr;
+		*lastentry = entryptr;
 		entrycount++;
 
-		next_offset = le32_to_cpu(entryptr->NextEntryOffset);
+		next_offset = le32_to_cpu(dir_info->NextEntryOffset);
 		if (!next_offset)
 			break;
 	}