Re: [PATCH] ipv4: ip_check_defrag must not modify skb before unsharing

From: Johannes Berg <redacted>
Date: Mon, 10 Dec 2012 19:45:52 +0100

On Mon, 2012-12-10 at 13:41 -0500, David Miller wrote:

quoted

So the bug is that ip_check_defrag() has a precondition which is met
properly by all callers except AF_PACKET.

If this is the case, remind me why are we changing ip_check_defrag()
rather than the violator of the precondition?

I don't think this is the case.

If you're referring to my note about af_packet: the kernels where this
goes into af_packet.c are the kernels that don't even have
ip_check_defrag() because macvlan didn't exist/didn't have ip defrag
support and af_packet had this code there -- see commit bc416d9768a.

If you're not referring to my note about af_packet: both callers (there
are only two) of ip_check_defrag() have this bug as far as I can tell
because they're both in the part of the RX path where shared SKBs might
happen.

You're right, I misinterpreted what's happening here.

My misunderstanding was that this was a situation where normal IPV4
input processing makes sure the SKB is unshared, and we had special
code paths that didn't make sure that was the case.

Rather, here, we have a special entrypoint for macvlan and AF_PACKET
which is supposed to take care of such issues since it is known to
execute in a different kind of environment.

I'm pretty sure I'll apply this, after I check a few more things,
thanks Johannes!
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