Re: [PATCH v2 2/2] kho: make sure page being restored is actually from KHO

[Mike Rapoport <rppt@kernel.org>]

On Wed, Sep 17, 2025 at 02:56:54PM +0200, Pratyush Yadav wrote:
> When restoring a page, no sanity checks are done to make sure the page
> actually came from a kexec handover. The caller is trusted to pass in
> the right address. If the caller has a bug and passes in a wrong
> address, an in-use page might be "restored" and returned, causing all
> sorts of memory corruption.
> 
> Harden the page restore logic by stashing in a magic number in
> page->private along with the order. If the magic number does not match,
> the page won't be touched. page->private is an unsigned long. The union
> kho_page_info splits it into two parts, with one holding the order and
> the other holding the magic number.
> 
> Signed-off-by: Pratyush Yadav <pratyush@kernel.org>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> 
> Notes:
>     Changes in v2:
>     
>     - Add a WARN_ON_ONCE() if order or magic is invalid.
>     - Add a comment explaining why the magic check also implicitly makes
>       sure phys is order-aligned.
>     - Clear page private to make sure later restores of the same page error
>       out.
>     - Move the checks to kho_restore_page() since patch 1 now moves sanity
>       checking to it.
> 
>  kernel/kexec_handover.c | 41 ++++++++++++++++++++++++++++++++++-------
>  1 file changed, 34 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
> index 69cab82abaaef..911fda8532b2e 100644
> --- a/kernel/kexec_handover.c
> +++ b/kernel/kexec_handover.c
> @@ -32,6 +32,22 @@
>  #define PROP_PRESERVED_MEMORY_MAP "preserved-memory-map"
>  #define PROP_SUB_FDT "fdt"
>  
> +#define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
> +
> +/*
> + * KHO uses page->private, which is an unsigned long, to store page metadata.
> + * Use it to store both the magic and the order.
> + */
> +union kho_page_info {
> +	unsigned long page_private;
> +	struct {
> +		unsigned int order;
> +		unsigned int magic;
> +	};
> +};
> +
> +static_assert(sizeof(union kho_page_info) == sizeof(((struct page *)0)->private));
> +
>  static bool kho_enable __ro_after_init;
>  
>  bool kho_is_enabled(void)
> @@ -186,16 +202,24 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
>  static struct page *kho_restore_page(phys_addr_t phys)
>  {
>  	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
> -	unsigned int nr_pages, order;
> +	union kho_page_info info;
> +	unsigned int nr_pages;
>  
>  	if (!page)
>  		return NULL;
>  
> -	order = page->private;
> -	if (order > MAX_PAGE_ORDER)
> +	info.page_private = page->private;
> +	/*
> +	 * deserialize_bitmap() only sets the magic on the head page. This magic
> +	 * check also implicitly makes sure phys is order-aligned since for
> +	 * non-order-aligned phys addresses, magic will never be set.
> +	 */
> +	if (WARN_ON_ONCE(info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER))
>  		return NULL;
> -	nr_pages = (1 << order);
> +	nr_pages = (1 << info.order);
>  
> +	/* Clear private to make sure later restores on this page error out. */
> +	page->private = 0;
>  	/* Head page gets refcount of 1. */
>  	set_page_count(page, 1);
>  
> @@ -203,8 +227,8 @@ static struct page *kho_restore_page(phys_addr_t phys)
>  	for (unsigned int i = 1; i < nr_pages; i++)
>  		set_page_count(page + i, 0);
>  
> -	if (order > 0)
> -		prep_compound_page(page, order);
> +	if (info.order > 0)
> +		prep_compound_page(page, info.order);
>  
>  	adjust_managed_page_count(page, nr_pages);
>  	return page;
> @@ -341,10 +365,13 @@ static void __init deserialize_bitmap(unsigned int order,
>  		phys_addr_t phys =
>  			elm->phys_start + (bit << (order + PAGE_SHIFT));
>  		struct page *page = phys_to_page(phys);
> +		union kho_page_info info;
>  
>  		memblock_reserve(phys, sz);
>  		memblock_reserved_mark_noinit(phys, sz);
> -		page->private = order;
> +		info.magic = KHO_PAGE_MAGIC;
> +		info.order = order;
> +		page->private = info.page_private;
>  	}
>  }
>  
> -- 
> 2.47.3
> 

-- 
Sincerely yours,
Mike.