fix some validation checks in dns response parsing code

since the buffer passed always has an actual size of 512 bytes, the
maximum possible response packet size, no out-of-bounds access was
possible; however, reading past the end of the valid portion of the
packet could cause the parser to attempt to process junk as answer
content.

diff --git a/src/network/dns_parse.c b/src/network/dns_parse.c
index aa0d39f39fbc85afea25a483297704d1c2267d78..0c7a60112473974601e86ac8fb0712dcb5fe3083 100644 (file)
--- a/src/network/dns_parse.c
+++ b/src/network/dns_parse.c
@@ -6,6 +6,7 @@ int __dns_parse(const unsigned char *r, int rlen, int (*callback)(void *, int, c
        const unsigned char *p;
        int len;
 
+       if (rlen<12) return -1;
        if ((r[3]&15)) return 0;
        p = r+12;
        qdcount = r[4]*256 + r[5];
@@ -13,13 +14,13 @@ int __dns_parse(const unsigned char *r, int rlen, int (*callback)(void *, int, c
        if (qdcount+ancount > 64) return -1;
        while (qdcount--) {
                while (p-r < rlen && *p-1U < 127) p++;
-               if (*p>193 || (*p==193 && p[1]>254) || p>r+506)
+               if (*p>193 || (*p==193 && p[1]>254) || p>r+rlen-6)
                        return -1;
                p += 5 + !!*p;
        }
        while (ancount--) {
                while (p-r < rlen && *p-1U < 127) p++;
-               if (*p>193 || (*p==193 && p[1]>254) || p>r+506)
+               if (*p>193 || (*p==193 && p[1]>254) || p>r+rlen-6)
                        return -1;
                p += 1 + !!*p;
                len = p[8]*256 + p[9];