netdev - Re: [PATCH net] ipv6: fix udpv6 sendmsg crash caused by too small MTU

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Message-ID: <CAJegeYn=OTLXXG3q6d64ozjOYO+HFy5DkHWwXs2w1oyTx7-uVg@mail.gmail.com>
Date:   Wed, 10 Jan 2018 12:43:13 -0500
From:   Mike Maloney <maloneykernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Mike Maloney <maloney@...gle.com>
Subject: Re: [PATCH net] ipv6: fix udpv6 sendmsg crash caused by too small MTU

D'oh - resending with fixes.  Thanks Eric.

On Wed, Jan 10, 2018 at 12:21 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2018-01-10 at 12:10 -0500, Mike Maloney wrote:
>> From: Mike Maloney <maloney@...gle.com>
>>
>> The logic in __ip6_append_data() assumes that the MTU is at least large
>> enough for the headers.  A device's MTU may be adjusted after being
>> added while sendmsg() is processing data, resulting in
>> __ip6_append_data() seeing any MTU.  For an mtu smaller than the size of
>> the fragmentation header, the math results in a negative 'maxfraglen',
>> which causes problems when refragmenting any previous skb in the
>> skb_write_queue, leaving it possibly malformed.
>>
>> Instead sendmsg returns EINVAL when the mtu is calculated to be less
>> than IPV6_MIN_MTU.
>>
>
> You forgot your SOB
>
>> Reported-by: syzbot <syzkaller@...glegroups.com>
>> ---
>
> Also please add after this '---' marker that your patch depends on my
> prior fix ( https://patchwork.ozlabs.org/patch/858234/
> ipv6: fix possible mem leaks in ipv6_make_skb() )
>
> ( Or we leak a dst reference )
>
> We probably should have sent a patch series.
>
> Thanks.
>
>>  net/ipv6/ip6_output.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>> index f7dd51c42314..f309ce7120d0 100644
>> --- a/net/ipv6/ip6_output.c
>> +++ b/net/ipv6/ip6_output.c
>> @@ -1206,14 +1206,16 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
>>       v6_cork->tclass = ipc6->tclass;
>>       if (rt->dst.flags & DST_XFRM_TUNNEL)
>>               mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
>> -                   rt->dst.dev->mtu : dst_mtu(&rt->dst);
>> +                   READ_ONCE(rt->dst.dev->mtu) : dst_mtu(&rt->dst);
>>       else
>>               mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
>> -                   rt->dst.dev->mtu : dst_mtu(rt->dst.path);
>> +                   READ_ONCE(rt->dst.dev->mtu) : dst_mtu(rt->dst.path);
>>       if (np->frag_size < mtu) {
>>               if (np->frag_size)
>>                       mtu = np->frag_size;
>>       }
>> +     if (mtu < IPV6_MIN_MTU)
>> +             return -EINVAL;
>>       cork->base.fragsize = mtu;
>>       if (dst_allfrag(rt->dst.path))
>>               cork->base.flags |= IPCORK_ALLFRAG;