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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Wed, 17 Feb 2021 18:37:58 -0500
Message-ID: <CAF=yD-LOF116aHub6RMe8vB8ZpnrrnoTdqhobEx+bvoA8AsP0w@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 6:18 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> On 2/18/21, Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
> > 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.
> >
>
> Bingo! Nice eye! I confirmed the crash by just memsetting 0x41 to cb
> before the call. Clearly this should be zeroed by icmp_ndo_xmit. Will
> send a patch for icmp_ndo_xmit momentarily and will CC you.

Great, let's hope that's it.

gtp_build_skb_ip4 zeroes before calling. The fix will be most
obviously correct if wg_xmit does the same.

But it is quite likely that the other callers, xfrmi_xmit2 and
sunvnet_start_xmit_common should zero, too. If so, then icmp_ndo_xmit
is the more robust location to do this. Then the Fixes tag will likely
go quite a bit farther back, too.

Whichever variant of the patch you prefer.