[PATCH 3.2 121/126] tcp: take care of truncations done by sk_filter()

Ben Hutchings
2017-02-15T22:52:50Z

From: Eric Dumazet <edumazet@google.com>

commit ac6e780070e30e4c35bd395acfe9191e6268bdd3 upstream.

With syzkaller help, Marco Grassi found a bug in TCP stack,
crashing in tcp_collapse()

Root cause is that sk_filter() can truncate the incoming skb,
but TCP stack was not really expecting this to happen.
It probably was expecting a simple DROP or ACCEPT behavior.

We first need to make sure no part of TCP header could be removed.
Then we need to adjust TCP_SKB_CB(skb)->end_seq

Many thanks to syzkaller team and Marco for giving us a reproducer.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Marco Grassi <marco.gra@gmail.com>
Reported-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
include/net/tcp.h | 1 +
net/ipv4/tcp_ipv4.c | 19 ++++++++++++++++++-
net/ipv6/tcp_ipv6.c | 6 ++++--
3 files changed, 23 insertions(+), 3 deletions(-)

--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -966,6 +966,7 @@ static inline int tcp_prequeue(struct so
return 1;
}

+int tcp_filter(struct sock *sk, struct sk_buff *skb);

#undef STATE_TRACE

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1647,6 +1647,21 @@ csum_err:
}
EXPORT_SYMBOL(tcp_v4_do_rcv);

+int tcp_filter(struct sock *sk, struct sk_buff *skb)
+{
+ struct tcphdr *th = (struct tcphdr *)skb->data;
+ unsigned int eaten = skb->len;
+ int err;
+
+ err = sk_filter_trim_cap(sk, skb, th->doff * 4);
+ if (!err) {
+ eaten -= skb->len;
+ TCP_SKB_CB(skb)->end_seq -= eaten;
+ }
+ return err;
+}
+EXPORT_SYMBOL(tcp_filter);
+
/*
* From tcp_input.c
*/
@@ -1709,8 +1724,10 @@ process:
goto discard_and_relse;
nf_reset(skb);

- if (sk_filter(sk, skb))
+ if (tcp_filter(sk, skb))
goto discard_and_relse;
+ th = (const struct tcphdr *)skb->data;
+ iph = ip_hdr(skb);

skb->dev = NULL;

--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1585,7 +1585,7 @@ static int tcp_v6_do_rcv(struct sock *sk
goto discard;
#endif

- if (sk_filter(sk, skb))
+ if (tcp_filter(sk, skb))
goto discard;

/*
@@ -1743,8 +1743,10 @@ process:
if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
goto discard_and_relse;

- if (sk_filter(sk, skb))
+ if (tcp_filter(sk, skb))
goto discard_and_relse;
+ th = (const struct tcphdr *)skb->data;
+ hdr = ipv6_hdr(skb);

skb->dev = NULL;