netdev - Re: [PATCH] cls_u32: use skb_copy_bits() to dereference data safely

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Date:	Wed, 02 Jun 2010 05:47:36 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	hadi@...erus.ca
Cc:	xiaosuo@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] cls_u32: use skb_copy_bits() to dereference data safely

From: jamal <hadi@...erus.ca>
Date: Wed, 02 Jun 2010 08:20:19 -0400

> I am suprised we never caught this all this years and wondering why
> this never crashed in your example?

Well for one thing there is all sorts of "stuff" past the end of the
valid skb->data area.  For example, there is some padding and then
there is skb_shared_info().

Furthermore, the kernel allocator can round up the size it uses for
SLAB objects which gives even more padding past the end of even
skb_shared_info().

Futrhermore, the chance of the page past the page skb->data is in
being invalid is very low.  You'd have to have invalid memory in the
page after the skb->data.

All of this conspires to just letting blind reads work in a large
number of illegal cases.
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