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

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Date:   Tue, 24 Oct 2017 09:58:48 -0700
From:   Jonathan Basseri 😶 <misterikkit@...gle.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     netdev@...r.kernel.org, Jakub Sitnicki <jkbs@...hat.com>,
        David Miller <davem@...emloft.net>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net] xfrm: Clear sk_dst_cache when applying per-socket policy.

On Tue, Oct 24, 2017 at 12:04 AM, Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> On Mon, Oct 23, 2017 at 06:18:55PM -0700, Jonathan Basseri 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.
>
> I think we don't have this problem with 'normal' policies. When
> inserting such a policy, we bump the IPv4/IPv6 genid. This should
> invalidate all cached dst entries, no?
>
That sounds reasonable to me. I had not confirmed the behavior for
normal policies, so I was trying to point out that this fix is only
for socket policies. Should I modify the commit message?