netdev - Re: possible stack corruption in icmp_send (__stack_chk

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Date:   Thu, 18 Feb 2021 00:18:19 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: possible stack corruption in icmp_send (__stack_chk_fail)

On 2/18/21, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> On Wed, Feb 17, 2021 at 5:56 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>>
>> Hi Willem,
>>
>> On Wed, Feb 17, 2021 at 11:27 PM Willem de Bruijn
>> <willemdebruijn.kernel@...il.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.

Jason