QDnsLookup/Unix: make sure we don't overflow the buffer

The DNS Records are variable length and encode their size in 16 bits
before the Record Data (RDATA). Ensure that both the RDATA and the
Record header fields before it fall inside the buffer we have.

Additionally reject any replies containing more than one query records.

[ChangeLog][QtNetwork][QDnsLookup] Fixed a bug that could cause a buffer
overflow in Unix systems while parsing corrupt, malicious, or truncated
replies.

Pick-to: 5.15 6.2 6.5 6.5.1
Change-Id: I3e3bfef633af4130a03afffd175e4b9547654b95
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

1 files changed, 25 insertions, 6 deletions

diff --git a/src/network/kernel/qdnslookup_unix.cpp b/src/network/kernel/qdnslookup_unix.cpp
index bb1c4feb95..ceab943a04 100644
--- a/src/network/kernel/qdnslookup_unix.cpp
+++ b/src/network/kernel/qdnslookup_unix.cpp
@@ -98,7 +98,6 @@ void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestN
     // responseLength in case of error, we still can extract the
     // exact error code from the response.
     HEADER *header = (HEADER*)response;
-    const int answerCount = ntohs(header->ancount);
     switch (header->rcode) {
     case NOERROR:
         break;
@@ -132,18 +131,31 @@ void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestN
         return;
     }
 
-    // Skip the query host, type (2 bytes) and class (2 bytes).
     char host[PACKETSZ], answer[PACKETSZ];
     unsigned char *p = response + sizeof(HEADER);
-    int status = dn_expand(response, response + responseLength, p, host, sizeof(host));
-    if (status < 0) {
+    int status;
+
+    if (ntohs(header->qdcount) == 1) {
+        // Skip the query host, type (2 bytes) and class (2 bytes).
+        status = dn_expand(response, response + responseLength, p, host, sizeof(host));
+        if (status < 0) {
+            reply->error = QDnsLookup::InvalidReplyError;
+            reply->errorString = tr("Could not expand domain name");
+            return;
+        }
+        if ((p - response) + status + 4 >= responseLength)
+            header->qdcount = 0xffff;   // invalid reply below
+        else
+            p += status + 4;
+    }
+    if (ntohs(header->qdcount) > 1) {
         reply->error = QDnsLookup::InvalidReplyError;
-        reply->errorString = tr("Could not expand domain name");
+        reply->errorString = tr("Invalid reply received");
         return;
     }
-    p += status + 4;
 
     // Extract results.
+    const int answerCount = ntohs(header->ancount);
     int answerIndex = 0;
     while ((p < response + responseLength) && (answerIndex < answerCount)) {
         status = dn_expand(response, response + responseLength, p, host, sizeof(host));
@@ -155,6 +167,11 @@ void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestN
         const QString name = QUrl::fromAce(host);
 
         p += status;
+
+        if ((p - response) + 10 > responseLength) {
+            // probably just a truncated reply, return what we have
+            return;
+        }
         const quint16 type = (p[0] << 8) | p[1];
         p += 2; // RR type
         p += 2; // RR class
@@ -162,6 +179,8 @@ void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestN
         p += 4;
         const quint16 size = (p[0] << 8) | p[1];
         p += 2;
+        if ((p - response) + size > responseLength)
+            return;             // truncated
 
         if (type == QDnsLookup::A) {
             if (size != 4) {