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Subject: Re: KASAN: use-after-free Read in __dev_queue_xmit
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Eric Biggers <ebiggers3@gmail.com>
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        syzbot <syzbot+71d74a5406d02057d559@syzkaller.appspotmail.com>,
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>,
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        Sowmini Varadhan <sowmini.varadhan@oracle.com>,
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        Willem de Bruijn <willemb@google.com>
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From:   Eric Dumazet <eric.dumazet@gmail.com>
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Date:   Wed, 9 May 2018 12:36:38 -0700
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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.