linux-kernel - Re: KASAN: use-after-free Read in __dev_queue

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Message-ID: <CAF=yD-LwxT6+9U-qCcYEVsPUeL+eVG_aSUZzEM7spwHGb7AQgQ@mail.gmail.com>
Date:   Wed, 9 May 2018 17:05:23 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Biggers <ebiggers3@...il.com>,
        syzbot <syzbot+71d74a5406d02057d559@...kaller.appspotmail.com>,
        alexander.deucher@....com,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Anoob Soman <anoob.soman@...rix.com>, chris@...is-wilson.co.uk,
        David Miller <davem@...emloft.net>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Maloney <maloney@...gle.com>, mchehab@...nel.org,
        netdev <netdev@...r.kernel.org>,
        "Rosen, Rami" <rami.rosen@...el.com>,
        Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        syzkaller-bugs@...glegroups.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: KASAN: use-after-free Read in __dev_queue_xmit

On Wed, May 9, 2018 at 3:36 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 05/09/2018 12:21 PM, Willem de Bruijn wrote:
>
>> Indeed. The skb shared info struct is zeroed by dev_validate_header
>> as a result of dev->hard_header_len exceeding skb->end - skb->data.
>>
>> Not exactly sure yet how this can happen. The hard header length space
>> is accounted for during allocation as reserved memory. But,
>> packet_alloc_skb does call skb_reserve(), moving skb->data
>> effectively beyond this reserved region.
>>
>> It may be incorrect to pass skb->data to dev_validate_header, as that
>> does not point to the start of the ll_header anymore. Still figuring out what
>> the right fix is..
>>
>
> I believe the bug happens if the sock_wmalloc() call at line 1921 has to sleep.
>
> device can change (or at lest dev->hard_header_len can change)
>
> So we need to bailout if reserved/hhlen had changed.
>
> Or revert some patches, since dev_hold() and dev_put() are no longer high cost,
> since it is now using per cpu counter.

Oh nice, another bug :/

That seems quite plausible.

This reproducer does not modify hard_header_len, however.
It sends a long array of zero byte requests with sendmmsg to
eventually exceed so_rcvbuf of the error queue. Hard header
length is 116 throughout.