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: <CALzav=egQiF6tanYxR9Tow7TnT_UK9bNAR_4DQ2P=2EJ+H4ZJA@mail.gmail.com>
Date: Mon, 27 Oct 2025 15:29:05 -0700
From: David Matlack <dmatlack@...gle.com>
To: Pasha Tatashin <pasha.tatashin@...een.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 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).

>  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.