Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct

[Kees Cook <keescook@chromium.org>]

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