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Date:   Fri, 23 Sep 2022 11:07:59 +0200
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Subject: Re: [PATCH 00/12] slab: Introduce kmalloc_size_roundup()
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To:     Kees Cook <keescook@chromium.org>
Cc:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Pekka Enberg <penberg@kernel.org>,
        Feng Tang <feng.tang@intel.com>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Alex Elder <elder@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Daniel Micay <danielmicay@gmail.com>,
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        linux-btrfs@vger.kernel.org, linux-media@vger.kernel.org,
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        linux-wireless@vger.kernel.org, llvm@lists.linux.dev,
        linux-hardening@vger.kernel.org,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>
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From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <202209221446.5E90AEED@keescook>
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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().