linux-kernel - Re: [RFC][PATCH 00/13] Provide saturating helpers for allocation

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Message-ID: <CAGXu5jKXq7--CYgp8Q+k00RjjGJY+o71RMr51NPuWS1eM0KX1w@mail.gmail.com>
Date:   Wed, 9 May 2018 10:01:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Matthew Wilcox <mawilcox@...rosoft.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC][PATCH 00/13] Provide saturating helpers for allocation

On Wed, May 9, 2018 at 9:08 AM, Laura Abbott <labbott@...hat.com> wrote:
> On 05/08/2018 05:42 PM, Kees Cook wrote:
>>
>> This is a stab at providing three new helpers for allocation size
>> calculation:
>>
>> struct_size(), array_size(), and array3_size().
>>
>> These are implemented on top of Rasmus's overflow checking functions,
>> and the last 8 patches are all treewide conversions of open-coded
>> multiplications into the various combinations of the helper functions.
>>
>> -Kees
>>
>>
> Obvious question (that might indicate this deserves documentation?)
>
> What's the difference between
>
> kmalloc_array(cnt, sizeof(struct blah), GFP_KERNEL);
>
> and
>
> kmalloc(array_size(cnt, struct blah), GFP_KERNEL);
>
>
> and when would you use one over the other?

If I'm understanding the intentions here, the next set of treewide
changes would be to remove *calloc() and *_array() in favor of using
the array_size() helper. (i.e. reducing proliferation of allocator
helpers in favor of using the *_size() helpers.

There are, however, some cases that don't map well to
{struct,array,array3}_size(), specifically cases of additions in
finding a count. For example, stuff like:

kmalloc(sizeof(header) + sizeof(trailing_array) * (count + SOMETHING), gfp...)

This gets currently mapped to:

kmalloc(struct_size(header, trailing_array, (count + SOMETHING), gfp...)

But we run the risk in some cases of having even the addition
overflow. I think we need to have a "saturating add" too. Something
like:

kmalloc(struct_size(header, trailing_array, sat_add(count, SOMETHING), gfp...)

It's a bit ugly, but it would cover nearly all the remaining cases...

-Kees

-- 
Kees Cook
Pixel Security