netdev - Re: [PATCH net] tcp: don't read out-of-bounds opsize

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Message-ID: <552d49b6-1b6e-c320-b56a-a119e360f1d7@gmail.com>
Date:   Fri, 20 Apr 2018 07:21:35 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jann Horn <jannh@...gle.com>, davem@...emloft.net,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] tcp: don't read out-of-bounds opsize



On 04/20/2018 06:57 AM, Jann Horn wrote:
> The old code reads the "opsize" variable from out-of-bounds memory (first
> byte behind the segment) if a broken TCP segment ends directly after an
> opcode that is neither EOL nor NOP.
> 
> The result of the read isn't used for anything, so the worst thing that
> could theoretically happen is a pagefault; and since the physmap is usually
> mostly contiguous, even that seems pretty unlikely.
>

No page fault possible, because tcp headers are in skb->head

And we have 'struct skb_shared_info'  at the end of skb->head anyway.

But, yes, reading some extra bytes with random content is possible.