BLASTing Linux Code

Jan Tobias Mühlberg and Gerald Lüttgen
Department of Computer Science, University of York, York YO10 5DD, U.K.


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Checking Memory Safety: Example 7


Commit Overview

Commit Key 703b69791369263e1d15f88f3e6aed02c1514fc2
Subject [PATCH] Fix another crash in ip_nat_pptp (CVE-2006-0037)
Description The PPTP NAT helper calculates the offset at which the packet needs to be mangled as difference between two pointers to the header. With non-linear skbs however the pointers may point to two seperate buffers on the stack and the calculation results in a wrong offset beeing used.
Requires Linux 2.6.14 kernel source as from git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/linux-2.6.14.y.git

--- a/net/ipv4/netfilter/ip_nat_helper_pptp.c
+++ b/net/ipv4/netfilter/ip_nat_helper_pptp.c
@@ -148,14 +148,14 @@ pptp_outbound_pkt(struct sk_buff **pskb,
{
struct ip_ct_pptp_master *ct_pptp_info = &ct->help.ct_pptp_info;
struct ip_nat_pptp *nat_pptp_info = &ct->nat.help.nat_pptp_info;
-
- u_int16_t msg, *cid = NULL, new_callid;
+ u_int16_t msg, new_callid;
+ unsigned int cid_off;
new_callid = htons(ct_pptp_info->pns_call_id);
switch (msg = ntohs(ctlh->messageType)) {
case PPTP_OUT_CALL_REQUEST:
- cid = &pptpReq->ocreq.callID;
+ cid_off = offsetof(union pptp_ctrl_union, ocreq.callID);
/* FIXME: ideally we would want to reserve a call ID
* here. current netfilter NAT core is not able to do
* this :( For now we use TCP source port. This breaks
@@ -172,10 +172,10 @@ pptp_outbound_pkt(struct sk_buff **pskb,
ct_pptp_info->pns_call_id = ntohs(new_callid);
break;
case PPTP_IN_CALL_REPLY:
- cid = &pptpReq->icreq.callID;
+ cid_off = offsetof(union pptp_ctrl_union, icreq.callID);
break;
case PPTP_CALL_CLEAR_REQUEST:
- cid = &pptpReq->clrreq.callID;
+ cid_off = offsetof(union pptp_ctrl_union, clrreq.callID);
break;
default:
DEBUGP("unknown outbound packet 0x%04x:%s\n", msg,
@@ -197,18 +197,15 @@ pptp_outbound_pkt(struct sk_buff **pskb,
/* only OUT_CALL_REQUEST, IN_CALL_REPLY, CALL_CLEAR_REQUEST pass
* down to here */
-
- IP_NF_ASSERT(cid);
-
DEBUGP("altering call id from 0x%04x to 0x%04x\n",
- ntohs(*cid), ntohs(new_callid));
+ ntohs(*(u_int16_t *)pptpReq + cid_off), ntohs(new_callid));
/* mangle packet */
if (ip_nat_mangle_tcp_packet(pskb, ct, ctinfo,
- (void *)cid - ((void *)ctlh - sizeof(struct pptp_pkt_hdr)),
- sizeof(new_callid),
- (char *)&new_callid,
- sizeof(new_callid)) == 0)
+ cid_off + sizeof(struct pptp_pkt_hdr) +
+ sizeof(struct PptpControlHeader),
+ sizeof(new_callid), (char *)&new_callid,
+ sizeof(new_callid)) == 0)
return NF_DROP;
return NF_ACCEPT;
@@ -297,7 +294,8 @@ pptp_inbound_pkt(struct sk_buff **pskb,
union pptp_ctrl_union *pptpReq)
{
struct ip_nat_pptp *nat_pptp_info = &ct->nat.help.nat_pptp_info;
- u_int16_t msg, new_cid = 0, new_pcid, *pcid = NULL, *cid = NULL;
+ u_int16_t msg, new_cid = 0, new_pcid;
+ unsigned int pcid_off, cid_off = 0;
int ret = NF_ACCEPT, rv;
@@ -305,23 +303,23 @@ pptp_inbound_pkt(struct sk_buff **pskb,
switch (msg = ntohs(ctlh->messageType)) {
case PPTP_OUT_CALL_REPLY:
- pcid = &pptpReq->ocack.peersCallID;
- cid = &pptpReq->ocack.callID;
+ pcid_off = offsetof(union pptp_ctrl_union, ocack.peersCallID);
+ cid_off = offsetof(union pptp_ctrl_union, ocack.callID);
break;
case PPTP_IN_CALL_CONNECT:
- pcid = &pptpReq->iccon.peersCallID;
+ pcid_off = offsetof(union pptp_ctrl_union, iccon.peersCallID);
break;
case PPTP_IN_CALL_REQUEST:
/* only need to nat in case PAC is behind NAT box */
return NF_ACCEPT;
case PPTP_WAN_ERROR_NOTIFY:
- pcid = &pptpReq->wanerr.peersCallID;
+ pcid_off = offsetof(union pptp_ctrl_union, wanerr.peersCallID);
break;
case PPTP_CALL_DISCONNECT_NOTIFY:
- pcid = &pptpReq->disc.callID;
+ pcid_off = offsetof(union pptp_ctrl_union, disc.callID);
break;
case PPTP_SET_LINK_INFO:
- pcid = &pptpReq->setlink.peersCallID;
+ pcid_off = offsetof(union pptp_ctrl_union, setlink.peersCallID);
break;
default:
@@ -343,25 +341,24 @@ pptp_inbound_pkt(struct sk_buff **pskb,
* WAN_ERROR_NOTIFY, CALL_DISCONNECT_NOTIFY pass down here */
/* mangle packet */
- IP_NF_ASSERT(pcid);
DEBUGP("altering peer call id from 0x%04x to 0x%04x\n",
- ntohs(*pcid), ntohs(new_pcid));
+ ntohs(*(u_int16_t *)pptpReq + pcid_off), ntohs(new_pcid));
- rv = ip_nat_mangle_tcp_packet(pskb, ct, ctinfo,
- (void *)pcid - ((void *)ctlh - sizeof(struct pptp_pkt_hdr)),
+ rv = ip_nat_mangle_tcp_packet(pskb, ct, ctinfo,
+ pcid_off + sizeof(struct pptp_pkt_hdr) +
+ sizeof(struct PptpControlHeader),
sizeof(new_pcid), (char *)&new_pcid,
sizeof(new_pcid));
if (rv != NF_ACCEPT)
return rv;
if (new_cid) {
- IP_NF_ASSERT(cid);
DEBUGP("altering call id from 0x%04x to 0x%04x\n",
- ntohs(*cid), ntohs(new_cid));
- rv = ip_nat_mangle_tcp_packet(pskb, ct, ctinfo,
- (void *)cid - ((void *)ctlh - sizeof(struct pptp_pkt_hdr)),
- sizeof(new_cid),
- (char *)&new_cid,
+ ntohs(*(u_int16_t *)pptpReq + cid_off), ntohs(new_cid));
+ rv = ip_nat_mangle_tcp_packet(pskb, ct, ctinfo,
+ cid_off + sizeof(struct pptp_pkt_hdr) +
+ sizeof(struct PptpControlHeader),
+ sizeof(new_cid), (char *)&new_cid,
sizeof(new_cid));
if (rv != NF_ACCEPT)
return rv;

(purple: line numbers and function names; red: line removed; green: line added)

Files

Unmodified sources

Comments

This bug is nicely explained above. It results from a bogous assumption made before computing a pointer. Pointer arithmetic is rather difficult to analyse and to verify in general. One way to deal with problems like this would be to do the verification in respect of a memory model. However, this bug is out of scope for BLAST.

Source: http://www.kernel.org



Jan Tobias Mühlberg, $Date$