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Date: Thu, 3 May 2018 17:40:34 -0700
From: Kees Cook <>
To: Rasmus Villemoes <>
Cc: Daniel Vetter <>, Matthew Wilcox <>, 
	Julia Lawall <>, Andrew Morton <>, 
	Matthew Wilcox <>, Linux-MM <>, 
	LKML <>, 
	Kernel Hardening <>,, 
	Himanshu Jha <>, 
	Linus Torvalds <>
Subject: Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct

On Thu, May 3, 2018 at 5:36 PM, Kees Cook <> wrote:
> On Thu, May 3, 2018 at 4:00 PM, Rasmus Villemoes
> <> wrote:
>> On 2018-05-01 19:00, Kees Cook wrote:
>>> On Mon, Apr 30, 2018 at 2:29 PM, Rasmus Villemoes
>>> <> 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:
>>> That's a very nice series. Why did it never get taken?
>> Well, nobody seemed particularly interested, and then
>> 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! :)

Whatever the case, I think we need to clean up all the kmalloc() math
anyway. As mentioned earlier, there are a handful of more complex
cases, but the vast majority are just A * B. I've put up a series here
now, and I'll send it out soon. I want to think more about 3-factor
products, addition, etc:

The commit logs need more details (i.e. about making constants the
second argument for optimal compiler results, etc), but there's a
Coccinelle-generated first pass.


Kees Cook
Pixel Security

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