linux-kernel - Re: [x86/uaccess] 5b710f34e1: kernel BUG at mm/usercopy.c:75!

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Message-ID: <CAGXu5jLSoF=JvvjYsdsFsPbUzrKMcAJK+7Q5P8EwCA2EQsQ7hA@mail.gmail.com>
Date:	Wed, 17 Aug 2016 14:34:37 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	kernel test robot <xiaolong.ye@...el.com>, LKP <lkp@...org>,
	LKML <linux-kernel@...r.kernel.org>,
	Valdis Kletnieks <valdis.kletnieks@...edu>
Subject: Re: [x86/uaccess] 5b710f34e1: kernel BUG at mm/usercopy.c:75!

On Wed, Aug 17, 2016 at 2:30 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Aug 17, 2016 at 2:25 PM, Kees Cook <keescook@...omium.org> wrote:
>>
>> I had forwarded this bug Rik's way since the page-cross checking was
>> suggested by him. I'm happy to drop it; it was a suggested improvement
>> that was suspected to be safe (none of the folks testing this ran into
>> it and we saw no report during its time in -next). I can prepare a
>> patch if there isn't a better way to detect this kind of allocation.
>> (FWIW, slab is handled separately.)
>
> I can't think of any sane way to notice it normally.
>
> Yes, with __GFP_COMPOUND you get the compound flag bits set, but as
> mentioned, that's a special case for the large page VM handling, and
> not applicable in general.
>
> Very few things do higher order allocations outside of slab and the
> task struct. But it does happen. Even fewer of those then have
> contents that might get copied to user space, but it clearly happens
> at least for _one_ case, and I can't convince myself that there might
> not be other cases too..

Yup, totally, I'll send a patch to remove this and Rik and I can
investigate re-adding it later.

-Kees

-- 
Kees Cook
Nexus Security