Re: [PATCH net v2] tcp: prevent fetching dst twice in early demux code

From: Eric Dumazet
Date: Mon Mar 23 2015 - 10:39:33 EST

On Mon, 2015-03-23 at 15:14 +0100, Michal Kubecek wrote:
> On s390x, gcc 4.8 compiles this part of tcp_v6_early_demux()
> struct dst_entry *dst = sk->sk_rx_dst;
> if (dst)
> dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);
> to code reading sk->sk_rx_dst twice, once for the test and once for
> the argument of ip6_dst_check() (dst_check() is inline). This allows
> ip6_dst_check() to be called with null first argument, causing a crash.
> Protect sk->sk_rx_dst access by READ_ONCE() both in IPv4 and IPv6
> TCP early demux code.
> Fixes: 41063e9dd119 ("ipv4: Early TCP socket demux.")
> Fixes: c7109986db3c ("ipv6: Early TCP socket demux")
> Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
> ---

Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Thanks Michal

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