linux-kernel - Re: [PATCH v1 04/13] kho: Verify deserialization status and fix FDT alignment access

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Message-ID: <mafs0tsyw7e0a.fsf@kernel.org>
Date: Fri, 14 Nov 2025 17:52:37 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org,  bhe@...hat.com,  rppt@...nel.org,
  jasonmiu@...gle.com,  arnd@...db.de,  coxu@...hat.com,
  dave@...ilevsky.ca,  ebiggers@...gle.com,  graf@...zon.com,
  kees@...nel.org,  linux-kernel@...r.kernel.org,
  kexec@...ts.infradead.org,  linux-mm@...ck.org
Subject: Re: [PATCH v1 04/13] kho: Verify deserialization status and fix FDT
 alignment access

On Fri, Nov 14 2025, Pasha Tatashin wrote:

> During boot, kho_restore_folio() relies on the memory map having been
> successfully deserialized. If deserialization fails or no map is present,
> attempting to restore the FDT folio is unsafe.
>
> Update kho_mem_deserialize() to return a boolean indicating success. Use
> this return value in kho_memory_init() to disable KHO if deserialization
> fails. Also, the incoming FDT folio is never used, there is no reason to
> restore it.
>
> Additionally, use memcpy() to retrieve the memory map pointer from the FDT.
> FDT properties are not guaranteed to be naturally aligned, and accessing
> a 64-bit value via a pointer that is only 32-bit aligned can cause faults.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 32 ++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index a4b33ca79246..83aca3b4af15 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -450,20 +450,28 @@ static void __init deserialize_bitmap(unsigned int order,
>  	}
>  }
>  
> -static void __init kho_mem_deserialize(const void *fdt)
> +/* Return true if memory was deserizlied */
> +static bool __init kho_mem_deserialize(const void *fdt)
>  {
>  	struct khoser_mem_chunk *chunk;
> -	const phys_addr_t *mem;
> +	const void *mem_ptr;
> +	u64 mem;
>  	int len;
>  
> -	mem = fdt_getprop(fdt, 0, PROP_PRESERVED_MEMORY_MAP, &len);
> -
> -	if (!mem || len != sizeof(*mem)) {
> +	mem_ptr = fdt_getprop(fdt, 0, PROP_PRESERVED_MEMORY_MAP, &len);
> +	if (!mem_ptr || len != sizeof(u64)) {
>  		pr_err("failed to get preserved memory bitmaps\n");
> -		return;
> +		return false;
>  	}
> +	/* FDT guarantees 32-bit alignment, have to use memcpy */
> +	memcpy(&mem, mem_ptr, len);

Perhaps get_unaligned(mem) would have been simpler?

> +
> +	chunk = mem ? phys_to_virt(mem) : NULL;
> +
> +	/* No preserved physical pages were passed, no deserialization */
> +	if (!chunk)
> +		return false;

Should we disallow all kho_restore_{folio,pages}() calls too if this
fails? Ideally those should never happen since kho_retrieve_subtree()
will fail, so maybe as a debug aid?

>  
> -	chunk = *mem ? phys_to_virt(*mem) : NULL;
>  	while (chunk) {
>  		unsigned int i;
>  
> @@ -472,6 +480,8 @@ static void __init kho_mem_deserialize(const void *fdt)
>  					   &chunk->bitmaps[i]);
>  		chunk = KHOSER_LOAD_PTR(chunk->hdr.next);
>  	}
> +
> +	return true;
>  }
>  
>  /*
> @@ -1377,16 +1387,12 @@ static void __init kho_release_scratch(void)
>  
>  void __init kho_memory_init(void)
>  {
> -	struct folio *folio;
> -
>  	if (kho_in.scratch_phys) {
>  		kho_scratch = phys_to_virt(kho_in.scratch_phys);
>  		kho_release_scratch();
>  
> -		kho_mem_deserialize(kho_get_fdt());
> -		folio = kho_restore_folio(kho_in.fdt_phys);
> -		if (!folio)
> -			pr_warn("failed to restore folio for KHO fdt\n");
> +		if (!kho_mem_deserialize(kho_get_fdt()))
> +			kho_in.fdt_phys = 0;

The folio restore does serve a purpose: it accounts for that folio in
the system's total memory. See the call to adjust_managed_page_count()
in kho_restore_page(). In practice, I don't think it makes much of a
difference, but I don't see why not.

>  	} else {
>  		kho_reserve_scratch();
>  	}

-- 
Regards,
Pratyush Yadav