CVE-2017-12902/Zephyr: Fix bounds checking.

Use ND_TTEST() rather than comparing against ndo->ndo_snapend ourselves;
it's easy to get the tests wrong.

Check for running out of packet data before checking for running out of
captured data, and distinguish between running out of packet data (which
might just mean "no more strings") and running out of captured data
(which means "truncated").

This fixes a buffer over-read discovered by Forcepoint's security
researchers Otto Airamo & Antti Levomäki.

Add a test using the capture file supplied by the reporter(s).

1 files changed, 30 insertions, 15 deletions

diff --git a/print-zephyr.c b/print-zephyr.c
index eb7e382b..735e273f 100644
--- a/print-zephyr.c
+++ b/print-zephyr.c
@@ -83,24 +83,34 @@ static const struct tok z_types[] = {
 static char z_buf[256];
 
 static const char *
-parse_field(netdissect_options *ndo, const char **pptr, int *len)
+parse_field(netdissect_options *ndo, const char **pptr, int *len, int *truncated)
 {
     const char *s;
 
-    if (*len <= 0 || !pptr || !*pptr)
-	return NULL;
-    if (*pptr > (const char *) ndo->ndo_snapend)
-	return NULL;
-
+    /* Start of string */
     s = *pptr;
-    while (*pptr <= (const char *) ndo->ndo_snapend && *len >= 0 && **pptr) {
+    /* Scan for the NUL terminator */
+    for (;;) {
+	if (*len == 0) {
+	    /* Ran out of packet data without finding it */
+	    return NULL;
+	}
+	if (!ND_TTEST(**pptr)) {
+	    /* Ran out of captured data without finding it */
+	    *truncated = 1;
+	    return NULL;
+	}
+	if (**pptr == '\0') {
+	    /* Found it */
+	    break;
+	}
+	/* Keep scanning */
 	(*pptr)++;
 	(*len)--;
     }
+    /* Skip the NUL terminator */
     (*pptr)++;
     (*len)--;
-    if (*len < 0 || *pptr > (const char *) ndo->ndo_snapend)
-	return NULL;
     return s;
 }
 
@@ -139,6 +149,7 @@ zephyr_print(netdissect_options *ndo, const u_char *cp, int length)
     int parselen = length;
     const char *s;
     int lose = 0;
+    int truncated = 0;
 
     /* squelch compiler warnings */
 
@@ -149,8 +160,9 @@ zephyr_print(netdissect_options *ndo, const u_char *cp, int length)
     z.sender = 0;
     z.recipient = 0;
 
-#define PARSE_STRING				\
-	s = parse_field(ndo, &parse, &parselen);	\
+#define PARSE_STRING						\
+	s = parse_field(ndo, &parse, &parselen, &truncated);	\
+	if (truncated) goto trunc;				\
 	if (!s) lose = 1;
 
 #define PARSE_FIELD_INT(field)			\
@@ -183,10 +195,8 @@ zephyr_print(netdissect_options *ndo, const u_char *cp, int length)
     PARSE_FIELD_INT(z.multi);
     PARSE_FIELD_STR(z.multi_uid);
 
-    if (lose) {
-	ND_PRINT((ndo, " [|zephyr] (%d)", length));
-	return;
-    }
+    if (lose)
+        goto trunc;
 
     ND_PRINT((ndo, " zephyr"));
     if (strncmp(z.version+4, "0.2", 3)) {
@@ -318,4 +328,9 @@ zephyr_print(netdissect_options *ndo, const u_char *cp, int length)
     ND_PRINT((ndo, " to %s", z_triple(z.class, z.inst, z.recipient)));
     if (*z.opcode)
 	ND_PRINT((ndo, " op %s", z.opcode));
+    return;
+
+trunc:
+    ND_PRINT((ndo, " [|zephyr] (%d)", length));
+    return;
 }