netdev - Re: [PATCH net] xfrm: Clear sk_dst_cache when applying per-socket policy.

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Date:   Mon, 23 Oct 2017 18:30:39 -0700
From:   Jonathan Basseri 😶 <misterikkit@...gle.com>
To:     netdev@...r.kernel.org
Cc:     Jakub Sitnicki <jkbs@...hat.com>,
        David Miller <davem@...emloft.net>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jonathan Basseri <misterikkit@...gle.com>
Subject: Re: [PATCH net] xfrm: Clear sk_dst_cache when applying per-socket policy.

On Mon, Oct 23, 2017 at 6:18 PM, Jonathan Basseri
<misterikkit@...gle.com> wrote:
> If a socket has a valid dst cache, then xfrm_lookup_route will get
> skipped. However, the cache is not invalidated when applying policy to a
> socket (i.e. IPV6_XFRM_POLICY). The result is that new policies are
> sometimes ignored on those sockets. (Note: This was broken for IPv4 and
> IPv6 at different times.)
>
> This can be demonstrated like so,
> 1. Create UDP socket.
> 2. connect() the socket.
> 3. Apply an outbound XFRM policy to the socket.
> 4. send() data on the socket.
>
> Packets will continue to be sent in the clear instead of matching an
> xfrm or returning a no-match error (EAGAIN). This affects calls to
> send() and not sendto().
>
> Invalidating the sk_dst_cache is necessary to correctly apply xfrm
> policies. Since we do this in xfrm_user_policy(), the sk_lock was
> already acquired in either do_ip_setsockopt() or do_ipv6_setsockopt(),
> and we may call __sk_dst_reset().
>
> Performance impact should be negligible, since this code is only called
> when changing xfrm policy, and only affects the socket in question.
>
> Note: Creating normal XFRM policies should have a similar effect on
> sk_dst_cache entries that match the policy, but that is not fixed in
> this patch.
>
> Fixes: 00bc0ef5880d ("ipv6: Skip XFRM lookup if dst_entry in socket cache is valid")
> Tested: https://android-review.googlesource.com/517555
> Tested: https://android-review.googlesource.com/418659
> Signed-off-by: Jonathan Basseri <misterikkit@...gle.com>
>
> diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
> index 12213477cd3a..1f5cee2269af 100644
> --- a/net/xfrm/xfrm_state.c
> +++ b/net/xfrm/xfrm_state.c
> @@ -2045,33 +2045,34 @@ EXPORT_SYMBOL(km_is_alive);
>  int xfrm_user_policy(struct sock *sk, int optname, u8 __user *optval, int optlen)
>  {
>         int err;
>         u8 *data;
>         struct xfrm_mgr *km;
>         struct xfrm_policy *pol = NULL;
>
>         if (optlen <= 0 || optlen > PAGE_SIZE)
>                 return -EMSGSIZE;
>
>         data = memdup_user(optval, optlen);
>         if (IS_ERR(data))
>                 return PTR_ERR(data);
>
>         err = -EINVAL;
>         rcu_read_lock();
>         list_for_each_entry_rcu(km, &xfrm_km_list, list) {
>                 pol = km->compile_policy(sk, optname, data,
>                                          optlen, &err);
>                 if (err >= 0)
>                         break;
>         }
>         rcu_read_unlock();
>
>         if (err >= 0) {
>                 xfrm_sk_policy_insert(sk, err, pol);
>                 xfrm_pol_put(pol);
> +               __sk_dst_reset(sk);
>                 err = 0;
>         }
>
>         kfree(data);
>         return err;
>  }
> --
> 2.15.0.rc0.271.g36b669edcc-goog
>

I discussed the concerns with Eric and I believe this addresses them.
(http://www.spinics.net/lists/netdev/msg449652.html)