linux-kernel - Re: [PATCH v3 1/3] liveupdate: kho: warn and fail on metadata or preserved memory in scratch area

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Message-ID: <CA+CK2bAibAPzTq+edRTXS9g7Cs0w-zCiSSrXUkoFAHe7=3C0QA@mail.gmail.com>
Date: Mon, 27 Oct 2025 20:01:59 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: David Matlack <dmatlack@...gle.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, corbet@....net, 
	graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, masahiroy@...nel.org, 
	ojeda@...nel.org, pratyush@...nel.org, rdunlap@...radead.org, rppt@...nel.org, 
	tj@...nel.org, jasonmiu@...gle.com, skhawaja@...gle.com
Subject: Re: [PATCH v3 1/3] liveupdate: kho: warn and fail on metadata or
 preserved memory in scratch area

On Mon, Oct 27, 2025 at 6:29 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Mon, Oct 20, 2025 at 5:08 PM Pasha Tatashin
> <pasha.tatashin@...een.com> wrote:
> >
> > It is invalid for KHO metadata or preserved memory regions to be located
> > within the KHO scratch area, as this area is overwritten when the next
> > kernel is loaded, and used early in boot by the next kernel. This can
> > lead to memory corruption.
> >
> > Adds checks to kho_preserve_* and KHO's internal metadata allocators
> > (xa_load_or_alloc, new_chunk) to verify that the physical address of the
> > memory does not overlap with any defined scratch region. If an overlap
> > is detected, the operation will fail and a WARN_ON is triggered. To
> > avoid performance overhead in production kernels, these checks are
> > enabled only when CONFIG_KEXEC_HANDOVER_DEBUG is selected.
>
> How many scratch regions are there in practice? Checking
> unconditionally seems like a small price to pay to avoid possible
> memory corruption. Especially since most KHO preservation should
> happen while the VM is still running (so does not have to by
> hyper-optimized).

The debug option can be enabled on production system as well, we have
some debug options enabled, but I do not see a reason to make this a
fixed cost that can add up; the runtime cost scares me, as we might be
using KHO preserve/unpreserve often once stateless KHO + slab
preservation is implemented during some allocations paths. Let's keep
it optional.

>
> >  static void *xa_load_or_alloc(struct xarray *xa, unsigned long index, size_t sz)
> >  {
> > -       void *elm, *res;
> > +       void *res = xa_load(xa, index);
> >
> > -       elm = xa_load(xa, index);
> > -       if (elm)
> > -               return elm;
> > +       if (res)
> > +               return res;
> > +
> > +       void *elm __free(kfree) = kzalloc(sz, GFP_KERNEL);
>
> nit: This breaks the local style of always declaring variables at the
> beginning of blocks.

I think this suggestion came from Mike, to me it looks alright, as it
is only part of the clean-up path.

Pasha