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: <mafs0v7k7fd76.fsf@kernel.org>
Date: Wed, 22 Oct 2025 12:22:37 +0200
From: Pratyush Yadav <pratyush@...nel.org>
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,  dmatlack@...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, Pasha Tatashin 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.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
[...]
> @@ -133,26 +135,26 @@ static struct kho_out kho_out = {
>  
>  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);
>  
> -	elm = kzalloc(sz, GFP_KERNEL);
>  	if (!elm)
>  		return ERR_PTR(-ENOMEM);
>  
> +	if (WARN_ON(kho_scratch_overlap(virt_to_phys(elm), sz)))
> +		return ERR_PTR(-EINVAL);
> +
>  	res = xa_cmpxchg(xa, index, NULL, elm, GFP_KERNEL);
>  	if (xa_is_err(res))
> -		res = ERR_PTR(xa_err(res));
> -
> -	if (res) {
> -		kfree(elm);
> +		return ERR_PTR(xa_err(res));
> +	else if (res)
>  		return res;
> -	}
>  
> -	return elm;
> +	return no_free_ptr(elm);

Super small nit: there exists return_ptr(p) which is a tiny bit neater
IMO but certainly not worth doing a new revision over. So,

Reviewed-by: Pratyush Yadav <pratyush@...nel.org>

[...]

-- 
Regards,
Pratyush Yadav