Re: [PATCH v6 00/10] liveupdate: Rework KHO for in-kernel users & Fix memory corruption

[Pasha Tatashin]

On Mon, Oct 20, 2025 at 4:34 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Sat, Oct 18, 2025 at 01:17:46PM -0400, Pasha Tatashin wrote:
> > This series addresses comments and combines into one the two
> > series [1] and [2], and adds review-bys.
> >
> > This series refactors the KHO framework to better support in-kernel
> > users like the upcoming LUO. The current design, which relies on a
> > notifier chain and debugfs for control, is too restrictive for direct
> > programmatic use.
> >
> > The core of this rework is the removal of the notifier chain in favor of
> > a direct registration API. This decouples clients from the shutdown-time
> > finalization sequence, allowing them to manage their preserved state
> > more flexibly and at any time.
> >
> > Also, this series fixes a memory corruption bug in KHO that occurs when
> > KFENCE is enabled.
> >
> > The root cause is that KHO metadata, allocated via kzalloc(), can be
> > randomly serviced by kfence_alloc(). When a kernel boots via KHO, the
> > early memblock allocator is restricted to a "scratch area". This forces
> > the KFENCE pool to be allocated within this scratch area, creating a
> > conflict. If KHO metadata is subsequently placed in this pool, it gets
> > corrupted during the next kexec operation.
> >
> > [1] https://lore.kernel.org/all/20251007033100.836886-1-pasha.tatashin@xxxxxxxxxx
> > [2] https://lore.kernel.org/all/20251015053121.3978358-1-pasha.tatashin@xxxxxxxxxx
> >
> > Mike Rapoport (Microsoft) (1):
> >   kho: drop notifiers
> >
> > Pasha Tatashin (9):
> >   kho: allow to drive kho from within kernel
> >   kho: make debugfs interface optional
> >   kho: add interfaces to unpreserve folios and page ranes
> >   kho: don't unpreserve memory during abort
> >   liveupdate: kho: move to kernel/liveupdate
> >   kho: move kho debugfs directory to liveupdate
> >   liveupdate: kho: warn and fail on metadata or preserved memory in scratch area
> >   liveupdate: kho: Increase metadata bitmap size to PAGE_SIZE
> >   liveupdate: kho: allocate metadata directly from the buddy allocator
>
> The fixes should go before the preparation for LUO or even better as a
> separate series.
>
> I've reread the LUO preparation patches and I don't think they are useful
> on their own. They introduce a couple of unused interfaces and I think it's
> better to have them along with the rest of LUO patches.

Pulling them out to apply fixes separately feels counterproductive,
especially since we agreed to add the new kexec_handover_debug.c file.
The most straightforward path is to build on what's already in -next.
Let's stick with the current approach.

Thanks,
Pasha

>
> --
> Sincerely yours,
> Mike.