tftpd: Add missing bounds checks

In send_[rw]rq(), we were using strlcpy() to avoid overflowing our
packet buffer, then failing to check the result and blithely advancing
our pointer by the full length.

Luckily, this code is only ever used by tftp(1), not tftpd(8).

MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D57075

libexec/tftpd/tftp-io.c

@@ -173,11 +173,11 @@ send_error(int peer, int error)
 int
 send_wrq(int peer, char *filename, char *mode)
 {
-	int n;
+	char buf[MAXPKTSIZE];
 	struct tftphdr *tp;
 	char *bp;
-	char buf[MAXPKTSIZE];
-	int size;
+	size_t len;
+	int n, size;
 
 	if (debug & DEBUG_PACKETS)
 		tftp_log(LOG_DEBUG, "Sending WRQ: filename: '%s', mode '%s'",
@@ -191,17 +191,17 @@ send_wrq(int peer, char *filename, char *mode)
 	size = offsetof(struct tftphdr, th_stuff);
 
 	bp = tp->th_stuff;
-	strlcpy(bp, filename, sizeof(buf) - size);
-	bp += strlen(filename);
-	*bp = 0;
-	bp++;
-	size += strlen(filename) + 1;
+	len = strlcpy(bp, filename, sizeof(buf) - size);
+	if (len >= sizeof(buf) - size)
+		goto overflow;
+	bp += len + 1;
+	size += len + 1;
 
-	strlcpy(bp, mode, sizeof(buf) - size);
-	bp += strlen(mode);
-	*bp = 0;
-	bp++;
-	size += strlen(mode) + 1;
+	len = strlcpy(bp, mode, sizeof(buf) - size);
+	if (len >= sizeof(buf) - size)
+		goto overflow;
+	bp += len + 1;
+	size += len + 1;
 
 	if (options_rfc_enabled)
 		size += make_options(peer, bp, sizeof(buf) - size);
@@ -213,6 +213,9 @@ send_wrq(int peer, char *filename, char *mode)
 		return (1);
 	}
 	return (0);
+overflow:
+	tftp_log(LOG_ERR, "%s: file name too long", __func__);
+	return (1);
 }
 
 /*
@@ -221,11 +224,11 @@ send_wrq(int peer, char *filename, char *mode)
 int
 send_rrq(int peer, char *filename, char *mode)
 {
-	int n;
+	char buf[MAXPKTSIZE];
 	struct tftphdr *tp;
 	char *bp;
-	char buf[MAXPKTSIZE];
-	int size;
+	size_t len;
+	int n, size;
 
 	if (debug & DEBUG_PACKETS)
 		tftp_log(LOG_DEBUG, "Sending RRQ: filename: '%s', mode '%s'",
@@ -239,17 +242,17 @@ send_rrq(int peer, char *filename, char *mode)
 	size = offsetof(struct tftphdr, th_stuff);
 
 	bp = tp->th_stuff;
-	strlcpy(bp, filename, sizeof(buf) - size);
-	bp += strlen(filename);
-	*bp = 0;
-	bp++;
-	size += strlen(filename) + 1;
+	len = strlcpy(bp, filename, sizeof(buf) - size);
+	if (len >= sizeof(buf) - size)
+		goto overflow;
+	bp += len + 1;
+	size += len + 1;
 
-	strlcpy(bp, mode, sizeof(buf) - size);
-	bp += strlen(mode);
-	*bp = 0;
-	bp++;
-	size += strlen(mode) + 1;
+	len = strlcpy(bp, mode, sizeof(buf) - size);
+	if (len >= sizeof(buf) - size)
+		goto overflow;
+	bp += len + 1;
+	size += len + 1;
 
 	if (options_rfc_enabled) {
 		options_set_request(OPT_TSIZE, "0");
@@ -263,6 +266,9 @@ send_rrq(int peer, char *filename, char *mode)
 		return (1);
 	}
 	return (0);
+overflow:
+	tftp_log(LOG_ERR, "%s: file name too long", __func__);
+	return (1);
 }
 
 /*