Re: [PATCH 00/12] slab: Introduce kmalloc_size_roundup()

Vlastimil Babka
2022-09-23T09:08:02Z

On 9/22/22 23:49, Kees Cook wrote:
> On Thu, Sep 22, 2022 at 11:05:47PM +0200, Vlastimil Babka wrote:
>> On 9/22/22 17:55, Kees Cook wrote:
>> > On Thu, Sep 22, 2022 at 09:10:56AM +0200, Christian König wrote:
>> > [...]
>> > > So when this patch set is about to clean up this use case it should probably
>> > > also take care to remove ksize() or at least limit it so that it won't be
>> > > used for this use case in the future.
>> >
>> > Yeah, my goal would be to eliminate ksize(), and it seems possible if
>> > other cases are satisfied with tracking their allocation sizes directly.
>>
>> I think we could leave ksize() to determine the size without a need for
>> external tracking, but from now on forbid callers from using that hint to
>> overflow the allocation size they actually requested? Once we remove the
>> kasan/kfence hooks in ksize() that make the current kinds of usage possible,
>> we should be able to catch any offenders of the new semantics that would appear?
>
> That's correct. I spent the morning working my way through the rest of
> the ksize() users I didn't clean up yesterday, and in several places I
> just swapped in __ksize(). But that wouldn't even be needed if we just
> removed the kasan unpoisoning from ksize(), etc.
>
> I am tempted to leave it __ksize(), though, just to reinforce that it's
> not supposed to be used "normally". What do you think?

Sounds good. Note in linux-next there's now a series in slab.git planned for
6.1 that moves __ksize() declaration to mm/slab.h to make it more private.
But we don't want random users outside mm and related kasan/kfence
subsystems to include mm/slab.h, so we'll have to expose it again instead of
ksize().