Re: [PATCH v1 1/2] kho: check scratch vs CMA alignment at runtime

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kho: bypassed scratch alignment check during resume causes memory corruption
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commit cbd7cb7f008e704b389d8dfe3306e1e83e49e8a8
Author: Sourabh Jain <sourabhjain@linux.ibm.com>

kho: check scratch vs CMA alignment at runtime

KHO requires the scratch alignment to be at least CMA_MIN_ALIGNMENT_BYTES,
which was previously enforced with a static_assert(). This commit replaces
the static_assert() with a runtime check in kho_reserve_scratch() to allow
KHO on configurations where the minimum alignment is not a constant.

> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 7c4d86daf86d5..dc76dd6b74da7 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c

[ ... ]

> @@ -836,6 +828,21 @@ static void __init kho_reserve_scratch(void)
>  	if (!kho_enable)
>  		return;
>  
> +	/*
> +	 * This is the minimal alignment required by deferred struct page init.
> +	 * deferred_init_memmap_chunk frees memory to the buddy allocator, which looks
> +	 * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
> +	 * If KHO scratch is not aligned to that value, buddy can access uninitialized
> +	 * struct pages, which can cause a crash.
> +	 */
> +	if (WARN_ON_ONCE(SCRATCH_ALIGNMENT_BYTES < CMA_MIN_ALIGNMENT_BYTES)) {
> +		pr_err("scratch alignment %lu < CMA minimum %lu, disabling\n",
> +		       (unsigned long)SCRATCH_ALIGNMENT_BYTES,
> +		       (unsigned long)CMA_MIN_ALIGNMENT_BYTES);
> +		kho_enable = false;
> +		return;
> +	}
> +

[Severity: High]
Does this new runtime check get bypassed during a KHO resume?

During a KHO resume, kho_memory_init() skips kho_reserve_scratch():

kernel/liveupdate/kexec_handover.c:kho_memory_init() {
	if (kho_in.scratch_phys)
		kho_mem_retrieve();
	else
		kho_reserve_scratch();
}

If a predecessor kernel passes a scratch region with a smaller alignment,
and the successor kernel has a larger pageblock_order, bypassing this check
means kho_init() will iterate over the provided scratch memory using the
successor kernel's larger pageblock_nr_pages:

kernel/liveupdate/kexec_handover.c:kho_init() {
		for (pfn = base_pfn; pfn < base_pfn + count;
		     pfn += pageblock_nr_pages)
			init_cma_reserved_pageblock(pfn_to_page(pfn));
}

Can this lead to init_cma_reserved_pageblock() overwriting state for pages
outside the actual scratch region bounds, or cause a crash in the buddy
allocator if __free_pages() is called on an unaligned PFN?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260928083226.107807-1-sourabhjain@linux.ibm.com?part=1