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

[Vlastimil Babka]

On 9/22/22 17:55, Kees Cook wrote:
> On Thu, Sep 22, 2022 at 09:10:56AM +0200, Christian König wrote:
>> Am 22.09.22 um 05:10 schrieb Kees Cook:
>> > Hi,
>> > 
>> > This series fixes up the cases where callers of ksize() use it to
>> > opportunistically grow their buffer sizes, which can run afoul of the
>> > __alloc_size hinting that CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE
>> > use to perform dynamic buffer bounds checking.
>> 
>> Good cleanup, but one question: What other use cases we have for ksize()
>> except the opportunistically growth of buffers?
> 
> The remaining cases all seem to be using it as a "do we need to resize
> yet?" check, where they don't actually track the allocation size
> themselves and want to just depend on the slab cache to answer it. This
> is most clearly seen in the igp code:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/intel/igb/igb_main.c?h=v6.0-rc6#n1204
> 
> My "solution" there kind of side-steps it, and leaves ksize() as-is:
> https://lore.kernel.org/linux-hardening/20220922031013.2150682-8-keescook@xxxxxxxxxxxx/
> 
> The more correct solution would be to add per-v_idx size tracking,
> similar to the other changes I sent:
> https://lore.kernel.org/linux-hardening/20220922031013.2150682-11-keescook@xxxxxxxxxxxx/
> 
> I wonder if perhaps I should just migrate some of this code to using
> something like struct membuf.
> 
>> Off hand I can't see any.
>> 
>> 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?

> -Kees
>