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

[Kees Cook]

On Mon, Apr 30, 2018 at 1:16 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Mon, Apr 30, 2018 at 12:02:14PM -0700, Kees Cook wrote:
>> For any longer multiplications, I've only found[1]:
>>
>> drivers/staging/rtl8188eu/os_dep/osdep_service.c:       void **a =
>> kzalloc(h * sizeof(void *) + h * w * size, GFP_KERNEL);
>
> That's pretty good, although it's just an atrocious vendor driver and
> it turns out all of those things are constants, and it'd be far better
> off with just declaring an array.  I bet they used to declare one on
> the stack ...

Yeah, it was just a quick hack to look for stuff.

>
>> At the end of the day, though, I don't really like having all these
>> different names...
>>
>> kmalloc(), kmalloc_array(), kmalloc_ab_c(), kmalloc_array_3d()
>>
>> with their "matching" zeroing function:
>>
>> kzalloc(), kcalloc(), kzalloc_ab_c(), kmalloc_array_3d(..., gfp | __GFP_ZERO)
>
> Yes, it's not very regular.
>
>> For the multiplication cases, I wonder if we could just have:
>>
>> kmalloc_multN(gfp, a, b, c, ...)
>> kzalloc_multN(gfp, a, b, c, ...)
>>
>> and we can replace all kcalloc() users with kzalloc_mult2(), all
>> kmalloc_array() users with kmalloc_mult2(), the abc uses with
>> kmalloc_mult3().
>
> I'm reluctant to do away with kcalloc() as it has the obvious heritage
> from user-space calloc() with the addition of GFP flags.

But it encourages misuse with calloc(N * M, gfp) ... if we removed
calloc and kept k[mz]alloc_something(gfp, a, b, c...) I think we'd
have better adoption.

>> That said, I *do* like kmalloc_struct() as it's a very common pattern...
>
> Thanks!  And way harder to misuse than kmalloc_ab_c().

Yes, quite so. It's really why I went with kmalloc_array_3d(), but now
I'm thinking better of it...

>> Or maybe, just leave the pattern in the name? kmalloc_ab(),
>> kmalloc_abc(), kmalloc_ab_c(), kmalloc_ab_cd() ?
>>
>> 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.

Yup, quite true. Obviously not the final form. ;) I meant to
illustrate that we could do compile-time tricks to reorder the
division in an efficient manner.

>> (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?

On the CPU it's not retained across multiple calculations. And the
type matters too. This came up recently in a separate thread too:
http://openwall.com/lists/kernel-hardening/2018/03/26/4

>> [1] git grep -E 'alloc\([^,]+[^(]\*[^)][^,]+[^(]\*[^)][^,]+[^(]\*[^)][^,]+,'
>
> I'm impressed, but it's not going to catch
>
>         veryLongPointerNameThatsMeaningfulToMe = kmalloc(initialSize +
>                 numberOfEntries * entrySize + someOtherThing * yourMum,
>                 GFP_KERNEL);

Right, it wasn't meant to be exhaustive. I just included it in case
anyone wanted to go grepping around for themselves.

-Kees

-- 
Kees Cook
Pixel Security