netdev - Re: [PATCH ipsec] xfrm: fix "disable_policy" flag use when arriving from different devices

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Date:   Thu, 12 May 2022 19:29:48 +0300
From:   Eyal Birger <eyal.birger@...il.com>
To:     nicolas.dichtel@...nd.com
Cc:     davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        steffen.klassert@...unet.com, herbert@...dor.apana.org.au,
        netdev@...r.kernel.org, Shmulik Ladkani <shmulik.ladkani@...il.com>
Subject: Re: [PATCH ipsec] xfrm: fix "disable_policy" flag use when arriving
 from different devices

On Thu, May 12, 2022 at 7:09 PM Nicolas Dichtel
<nicolas.dichtel@...nd.com> wrote:
>
>
> Le 12/05/2022 à 12:48, Eyal Birger a écrit :
> > In IPv4 setting the "disable_policy" flag on a device means no policy
> > should be enforced for traffic originating from the device. This was
> > implemented by seting the DST_NOPOLICY flag in the dst based on the
> > originating device.
> >
> > However, dsts are cached in nexthops regardless of the originating
> > devices, in which case, the DST_NOPOLICY flag value may be incorrect.
> >
> > Consider the following setup:
> >
> >                      +------------------------------+
> >                      | ROUTER                       |
> >   +-------------+    | +-----------------+          |
> >   | ipsec src   |----|-|ipsec0           |          |
> >   +-------------+    | |disable_policy=0 |   +----+ |
> >                      | +-----------------+   |eth1|-|-----
> >   +-------------+    | +-----------------+   +----+ |
> >   | noipsec src |----|-|eth0             |          |
> >   +-------------+    | |disable_policy=1 |          |
> >                      | +-----------------+          |
> >                      +------------------------------+
> >
> > Where ROUTER has a default route towards eth1.
> >
> > dst entries for traffic arriving from eth0 would have DST_NOPOLICY
> > and would be cached and therefore can be reused by traffic originating
> > from ipsec0, skipping policy check.
> >
> > Fix by setting a IPSKB_NOPOLICY flag in IPCB and observing it instead
> > of the DST in IN/FWD IPv4 policy checks.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Reported-by: Shmulik Ladkani <shmulik.ladkani@...il.com>
> > Signed-off-by: Eyal Birger <eyal.birger@...il.com>
> > ---
>
> [snip]
>
> > @@ -1852,8 +1856,7 @@ static int __mkroute_input(struct sk_buff *skb,
> >               }
> >       }
> >
> > -     rth = rt_dst_alloc(out_dev->dev, 0, res->type,
> > -                        IN_DEV_ORCONF(in_dev, NOPOLICY),
> > +     rth = rt_dst_alloc(out_dev->dev, 0, res->type, no_policy,>                         IN_DEV_ORCONF(out_dev, NOXFRM));
> no_policy / DST_NOPOLICY is still needed in the dst entry after this patch?

I see it's being set in the outbound direction in IPv4 - though I don't see
where it's actually used in that direction.

Maybe it could be cleaned up as a follow up, but I wanted to scope this
patch to the bugfix.

Eyal.