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tcp: take care of truncations done by sk_filter()

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>
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Eric Dumazet authored and davem330 committed Nov 10, 2016
1 parent 969447f commit ac6e780070e30e4c35bd395acfe9191e6268bdd3
Showing with 23 additions and 3 deletions.
  1. +1 −0 include/net/tcp.h
  2. +18 −1 net/ipv4/tcp_ipv4.c
  3. +4 −2 net/ipv6/tcp_ipv6.c
@@ -1220,6 +1220,7 @@ static inline void tcp_prequeue_init(struct tcp_sock *tp)

bool tcp_prequeue(struct sock *sk, struct sk_buff *skb);
bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb);
int tcp_filter(struct sock *sk, struct sk_buff *skb);

#undef STATE_TRACE

@@ -1564,6 +1564,21 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
}
EXPORT_SYMBOL(tcp_add_backlog);

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
*/
@@ -1676,8 +1691,10 @@ int tcp_v4_rcv(struct sk_buff *skb)

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;

@@ -1229,7 +1229,7 @@ static int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
if (skb->protocol == htons(ETH_P_IP))
return tcp_v4_do_rcv(sk, skb);

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

/*
@@ -1457,8 +1457,10 @@ static int tcp_v6_rcv(struct sk_buff *skb)
if (tcp_v6_inbound_md5_hash(sk, 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;

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