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Date: Tue, 1 May 2018 10:00:27 -0700
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>, Daniel Vetter <daniel.vetter@...el.com>
Cc: Matthew Wilcox <willy@...radead.org>, Julia Lawall <julia.lawall@...6.fr>, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <mawilcox@...rosoft.com>, 
	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, cocci@...teme.lip6.fr, 
	Himanshu Jha <himanshujha199640@...il.com>
Subject: Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct

On Mon, Apr 30, 2018 at 2:29 PM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> On 2018-04-30 22:16, Matthew Wilcox wrote:
>> On Mon, Apr 30, 2018 at 12:02:14PM -0700, Kees Cook wrote:
>>>
>>> Getting the constant ordering right could be part of the macro
>>> definition, maybe? i.e.:
>>>
>>> static inline void *kmalloc_ab(size_t a, size_t b, gfp_t flags)
>>> {
>>>     if (__builtin_constant_p(a) && a != 0 && \
>>>         b > SIZE_MAX / a)
>>>             return NULL;
>>>     else if (__builtin_constant_p(b) && b != 0 && \
>>>                a > SIZE_MAX / b)
>>>             return NULL;
>>>
>>>     return kmalloc(a * b, flags);
>>> }
>>
>> Ooh, if neither a nor b is constant, it just didn't do a check ;-(  This
>> stuff is hard.
>>
>>> (I just wish C had a sensible way to catch overflow...)
>>
>> Every CPU I ever worked with had an "overflow" bit ... do we have a
>> friend on the C standards ctte who might figure out a way to let us
>> write code that checks it?
>
> gcc 5.1+ (I think) have the __builtin_OP_overflow checks that should
> generate reasonable code. Too bad there's no completely generic
> check_all_ops_in_this_expression(a+b*c+d/e, or_jump_here). Though it's
> hard to define what they should be checked against - probably would
> require all subexpressions (including the variables themselves) to have
> the same type.
>
> plug: https://lkml.org/lkml/2015/7/19/358

That's a very nice series. Why did it never get taken? It seems to do
the right things quite correctly.

Daniel, while this isn't a perfect solution, is this something you'd
use in graphics-land?

-Kees

-- 
Kees Cook
Pixel Security

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