[PATCH 4.20 19/52] tcp: Dont access TCP_SKB_CB before initializing it

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christoph Paasch <cpaasch@apple.com>

[ Upstream commit f2feaefdabb0a6253aa020f65e7388f07a9ed47c ]

Since commit eeea10b83a13 ("tcp: add
tcp_v4_fill_cb()/tcp_v4_restore_cb()"), tcp_vX_fill_cb is only called
after tcp_filter(). That means, TCP_SKB_CB(skb)->end_seq still points to
the IP-part of the cb.

We thus should not mock with it, as this can trigger bugs (thanks
syzkaller):
[   12.349396] ==================================================================
[   12.350188] BUG: KASAN: slab-out-of-bounds in ip6_datagram_recv_specific_ctl+0x19b3/0x1a20
[   12.351035] Read of size 1 at addr ffff88006adbc208 by task test_ip6_datagr/1799

Setting end_seq is actually no more necessary in tcp_filter as it gets
initialized later on in tcp_vX_fill_cb.

Cc: Eric Dumazet <edumazet@google.com>
Fixes: eeea10b83a13 ("tcp: add tcp_v4_fill_cb()/tcp_v4_restore_cb()")
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_ipv4.c |    9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1646,15 +1646,8 @@ 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;
+	return sk_filter_trim_cap(sk, skb, th->doff * 4);
 }
 EXPORT_SYMBOL(tcp_filter);