Re: KASAN: use-after-free Read in __dev_queue_xmit

[Willem de Bruijn]

On Wed, May 9, 2018 at 3:36 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> 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.