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From:   Kees Cook <keescook@chromium.org>
Date:   Thu, 3 May 2018 17:36:28 -0700
Message-ID: <CAGXu5jJ9Uw9pDOYfBH8iXTVqiQXgNrEqzpk7a5mOCrH0G3CoyA@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Daniel Vetter <daniel.vetter@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Julia Lawall <julia.lawall@lip6.fr>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        cocci@systeme.lip6.fr, Himanshu Jha <himanshujha199640@gmail.com>
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On Thu, May 3, 2018 at 4:00 PM, Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
> On 2018-05-01 19:00, Kees Cook wrote:
>> On Mon, Apr 30, 2018 at 2:29 PM, Rasmus Villemoes
>> <linux@rasmusvillemoes.dk> wrote:
>>>
>>> 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?
>
> Well, nobody seemed particularly interested, and then
> https://lkml.org/lkml/2015/10/28/215 happened... but he did later seem
> to admit that it could be useful for the multiplication checking, and
> that "the gcc interface for multiplication overflow is fine".

Oh, excellent. Thank you for that pointer! That conversation covered a
lot of ground. I need to think a little more about how to apply the
thoughts there with the kmalloc() needs and the GPU driver needs...

> I still think even for unsigned types overflow checking can be subtle. E.g.
>
> u32 somevar;
>
> if (somevar + sizeof(foo) < somevar)
>   return -EOVERFLOW;
> somevar += sizeof(this);
>
> is broken, because the LHS is promoted to unsigned long/size_t, then so
> is the RHS for the comparison, and the comparison is thus always false
> (on 64bit). It gets worse if the two types are more "opaque", and in any
> case it's not always easy to verify at a glance that the types are the
> same, or at least that the expression of the widest type is on the RHS.

That's an excellent example, yes. (And likely worth including in the
commit log somewhere.)

>
>> It seems to do the right things quite correctly.
>
> Yes, I wouldn't suggest it without the test module verifying corner
> cases, and checking it has the same semantics whether used with old or
> new gcc.
>
> Would you shepherd it through if I updated the patches and resent?

Yes, though we may need reworking if we actually want to do the
try/catch style (since that was talked about with GPU stuff too...)

Either way, yes, a refresh would be lovely! :)

-Kees

-- 
Kees Cook
Pixel Security