Re: [PATCH v4] x86/kexec: fix potential cmem->ranges out of bounds

[Baoquan He <bhe@redhat.com>]

On 05/16/25 at 04:20pm, Kees Cook wrote:
> On Fri, May 16, 2025 at 11:35:12AM +0800, Baoquan He wrote:
> > On 05/11/25 at 10:19am, Coiby Xu wrote:
> > > On Fri, May 09, 2025 at 06:35:18PM -0700, Andrew Morton wrote:
> > > > On Fri, 9 May 2025 17:58:01 +0800 Baoquan He <bhe@redhat.com> wrote:
> > > > 
> > > > > > The bad commit was introduced in 2021 but only recent gcc-15 supports
> > > > > > __counted_by. That's why we don't see this UBSAN warning until this
> > > > > > year. And although this UBSAN warning is scary enough, fortunately it
> > > > > > doesn't cause a real problem.
> > > > > >
> > > > > > >
> > > > > > > Baoquan, please re-review this?
> > > > > > >
> > > > > > > A -stable backport is clearly required.  A Fixes: would be nice, but I
> > > > > > > assume this goes back a long time so it isn't worth spending a lot of
> > > > > > > time working out when this was introduced.
> > > > > >
> > > > > > So I believe the correct fix should be as follows,
> > > > > 
> > > > > Thanks for testing and investigation into these. Could you arrange this
> > > > > into formal patches based on your testing and analysis?
> > > > > 
> > > > > It would be great if you can include Fuqiang's patch since it has
> > > > > conflict with your LUKS patch. This can facilitate patch merging for
> > > > > Andrew. Thanks in advance.
> > > > 
> > > > Yes please, I'm a bit lost here.
> > > > x86-kexec-fix-potential-cmem-ranges-out-of-bounds.patch is not
> > > > presently in mm.git and I'd appreciate clarity on how to resolve the
> > > > conflicts which a new version of
> > > > x86-kexec-fix-potential-cmem-ranges-out-of-bounds.patch will produce.
> > > 
> > > I'll resolve any conflict between these patches. Before that, I'm not sure
> > > if a separate patch to fix the UBSAN warnings alone is needed to Cc
> > > stable@vger.kernel.org because 1) the UBSAN warnings don't mean there is a
> > > real problem;
> > > 2) both Fuqiang's patch and my kdump LUKS support patches fix the UBSAN
> > > warnings as a by-product.
> > > 
> > > It seems the answer largely depends on if the stable tree or longterm
> > > trees need it. Currently, only longterm tree 6.12.28 and the stable tree
> > > 6.14.6 have the UBSAN warnings if they are compiled with gcc-15 or
> > > clang-18. Any advice will be appreciated! Thanks!
> > 
> > I personally think UBSAN warning fix is not necessary for stable kernel.
> > 
> > Hi Kees, Andrew,
> > 
> > Could you help answer Coiby's question about whether we need post a
> > standalone patch to fix the UBSAN warning fix so that it can be back
> > ported to stable kernel?
> 
> I went back through the thread and the referenced threads and I can't
> find any details on the USBAN splat. Can that please get reproduced in a
> commit log? That would help understand if it's a false positive or not.


The original patch is trying to fix a potential issue in which a memory
range is split, while the sub-range split out is always on top of the
entire memory range, hence no risk.

Later, we encountered a UBSAN warning around the above memory range
splitting code several times. We found this patch can mute the warning.

Please see below UBSAN splat trace report from Coiby:
https://lore.kernel.org/all/4de3c2onosr7negqnfhekm4cpbklzmsimgdfv33c52dktqpza5@z5pb34ghz4at/T/#u

Later, Coiby got the root cause from investigation, please see:
https://lore.kernel.org/all/2754f4evjfumjqome63bc3inqb7ozepemejn2lcl57ryio2t6k@35l3tnn73gei/T/#u

> 
> Also, referencing the commit would be good. I assume this is discussing
> commit 15fcedd43a08 ("kexec: Annotate struct crash_mem with __counted_by")?

Right.

> 
> > In the case exposed during reviewing this patch, the code UBSAN warned
> > is not risky.
> 
> Given that this makes things work correctly with newer compilers, I
> would say it should be backported to whatever -stable kernels have the
> "counted_by" annotation. (Hence the request to add a "Fixes" line so
> that it will happen automatically.)

Got it, then Coiby can post a standalone patch to fix commit 15fcedd43a08
("kexec: Annotate struct crash_mem with __counted_by") and CC stable, then
post a new version of this patch on top.

Thanks a lot for confirming.