[PATCH v2 net] tcp: take care of truncations done by sk_filter()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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 <eduma...@google.com>
Reported-by: Marco Grassi <marco....@gmail.com>
Reported-by: Vladis Dronov <vdro...@redhat.com>
---
v2: reload both th/iph in case skb->head changed.

 include/net/tcp.h   |    1 +
 net/ipv4/tcp_ipv4.c |   19 ++++++++++++++++++-
 net/ipv6/tcp_ipv6.c |    6 ++++--
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 304a8e17bc87..123979fe12bf 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -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
 
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 61b7be303eec..2259114c7242 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -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;
 
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 6ca23c2e76f7..b9f1fee9a886 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -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;