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Date:   Thu, 22 Sep 2022 14:49:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christian König <christian.koenig@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alex Elder <elder@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Daniel Micay <danielmicay@...il.com>,
        Yonghong Song <yhs@...com>, Marco Elver <elver@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-fsdevel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        dev@...nvswitch.org, x86@...nel.org,
        linux-wireless@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH 00/12] slab: Introduce kmalloc_size_roundup()

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?

-- 
Kees Cook

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