CVE-2018-1065: NULL Pointer Dereference (CWE-476)
value: The netfilter subsystem in the Linux kernel through 4.15.7 mishandles the case of a rule blob that contains a jump but lacks a user-defined chain, which allows local users to cause a denial of service (NULL pointer dereference) by leveraging the CAP_NET_RAW or CAP_NET_ADMIN capability, related to arpt_do_table in net/ipv4/netfilter/arp_tables.c, ipt_do_table in net/ipv4/netfilter/ip_tables.c, and ip6t_do_table in net/ipv6/netfilter/ip6_tables.c.
CWE description: The product dereferences a pointer that it expects to be valid but is NULL.
CVSS v3 base score: 4.7
Published: 2018-03-02T08:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/ipv4/netfilter/arp_tables.c, net/ipv4/netfilter/ip_tables.c, net/ipv6/netfilter/ip6_tables.c
Commit date: 2018-02-14 20:47:41 +0100
Commit message: netfilter: add back stackpointer size checks

The rationale for removing the check is only correct for rulesets
generated by ip(6)tables.

In iptables, a jump can only occur to a user-defined chain, i.e.
because we size the stack based on number of user-defined chains we
cannot exceed stack size.

However, the underlying binary format has no such restriction,
and the validation step only ensures that the jump target is a
valid rule start point.

IOW, its possible to build a rule blob that has no user-defined
chains but does contain a jump.

If this happens, no jump stack gets allocated and crash occurs
because no jumpstack was allocated.

Fixes: 7814b6ec6d0d6 ("netfilter: xtables: don't save/restore jumpstack offset")
Reported-by: syzbot+e783f671527912cd9403@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Diff: diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
index 4ffe302f9b82..e3e420f3ba7b 100644
--- a/net/ipv4/netfilter/arp_tables.c
+++ b/net/ipv4/netfilter/arp_tables.c
@@ -250,10 +250,14 @@ unsigned int arpt_do_table(struct sk_buff *skb,
 				}
 				continue;
 			}
 			if (table_base + v
 			    != arpt_next_entry(e)) {
+				if (unlikely(stackidx >= private->stacksize)) {
+					verdict = NF_DROP;
+					break;
+				}
 				jumpstack[stackidx++] = e;
 			}
 
 			e = get_entry(table_base, v);
 			continue;
diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index 9a71f3149507..e38395a8dcf2 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -328,12 +328,17 @@ ipt_do_table(struct sk_buff *skb,
 					e = ipt_next_entry(e);
 				}
 				continue;
 			}
 			if (table_base + v != ipt_next_entry(e) &&
-			    !(e->ip.flags & IPT_F_GOTO))
+			    !(e->ip.flags & IPT_F_GOTO)) {
+				if (unlikely(stackidx >= private->stacksize)) {
+					verdict = NF_DROP;
+					break;
+				}
 				jumpstack[stackidx++] = e;
+			}
 
 			e = get_entry(table_base, v);
 			continue;
 		}
 
diff --git a/net/ipv6/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c
index af4c917e0836..62358b93bbac 100644
--- a/net/ipv6/netfilter/ip6_tables.c
+++ b/net/ipv6/netfilter/ip6_tables.c
@@ -350,10 +350,14 @@ ip6t_do_table(struct sk_buff *skb,
 					e = ip6t_next_entry(jumpstack[--stackidx]);
 				continue;
 			}
 			if (table_base + v != ip6t_next_entry(e) &&
 			    !(e->ipv6.flags & IP6T_F_GOTO)) {
+				if (unlikely(stackidx >= private->stacksize)) {
+					verdict = NF_DROP;
+					break;
+				}
 				jumpstack[stackidx++] = e;
 			}
 
 			e = get_entry(table_base, v);
 			continue;

Vulnerable code: 			    !(e->ip.flags & IPT_F_GOTO))
Fixed code: 				if (unlikely(stackidx >= private->stacksize)) {
					verdict = NF_DROP;
					break;
				}
			    !(e->ip.flags & IPT_F_GOTO)) {
				if (unlikely(stackidx >= private->stacksize)) {
					verdict = NF_DROP;
					break;
				}
			}
				if (unlikely(stackidx >= private->stacksize)) {
					verdict = NF_DROP;
					break;
				}