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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Wed, 17 Feb 2021 18:05:54 -0500
Message-ID: <CAF=yD-+Fm7TuggoEeP=Wy7DEmfuC6nxmyBQxX=EzhyTQsiP2DQ@mail.gmail.com>
Subject: Re: possible stack corruption in icmp_send (__stack_chk_fail)
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Willem de Bruijn <willemb@google.com>
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On Wed, Feb 17, 2021 at 5:56 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Willem,
>
> On Wed, Feb 17, 2021 at 11:27 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> > A vmlinux image might help. I couldn't find one for this kernel.
>
> https://data.zx2c4.com/icmp_send-crash-e03b4a42-706a-43bf-bc40-1f15966b3216.tar.xz
> has .debs with vmlinuz in there, which you can extract to vmlinux, as
> well as my own vmlinux elf construction with the symbols added back in
> by extracting them from kallsyms. That's the best I've been able to
> do, as all of this is coming from somebody random emailing me.
>
> > But could it be
> > that the forwarded packet is not sensible IPv4? The skb->protocol is
> > inferred in wg_packet_consume_data_done->ip_tunnel_parse_protocol.
>
> The wg calls to icmp_ndo_send are gated by checking skb->protocol:
>
>         if (skb->protocol == htons(ETH_P_IP))
>                icmp_ndo_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_UNREACH, 0);
>        else if (skb->protocol == htons(ETH_P_IPV6))
>                icmpv6_ndo_send(skb, ICMPV6_DEST_UNREACH,
> ICMPV6_ADDR_UNREACH, 0);
>
> On the other hand, that code is hit on an error path when
> wg_check_packet_protocol returns false:
>
> static inline bool wg_check_packet_protocol(struct sk_buff *skb)
> {
>        __be16 real_protocol = ip_tunnel_parse_protocol(skb);
>        return real_protocol && skb->protocol == real_protocol;
> }
>
> So that means, at least in theory, icmp_ndo_send could be called with
> skb->protocol != ip_tunnel_parse_protocol(skb). I guess I can address
> that. But... is it actually a problem?

For this forwarded packet that arrived on a wireguard tunnel,
skb->protocol was originally also set by ip_tunnel_parse_protocol.
So likely not.

The other issue seems more like a real bug. wg_xmit calling
icmp_ndo_send without clearing IPCB first.