Re: [PATCH] kho: validate order in deserialize_bitmap()

[Mike Rapoport <rppt@kernel.org>]

Hi Marco,

On Sat, Feb 14, 2026 at 01:57:51AM +0100, Marco Elver wrote:
> The function deserialize_bitmap() calculates the reservation size using:
> 
>     int sz = 1 << (order + PAGE_SHIFT);
> 
> If a corrupted KHO image provides an order >= 20 (on systems with 4KB
> pages), the shift amount becomes >= 32, which overflows the 32-bit
> integer. This results in a zero-size memory reservation.
> 
> Furthermore, the physical address calculation:
> 
>     phys_addr_t phys = elm->phys_start + (bit << (order + PAGE_SHIFT));
> 
> can also overflow and wrap around if the order is large. This allows a
> corrupt KHO image to cause out-of-bounds updates to page->private of
> arbitrary physical pages during early boot.
> 
> Fix this by adding a bounds check for the order field.
> 
> Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index b851b09a8e99..ec353e4b68a6 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -463,6 +463,11 @@ static void __init deserialize_bitmap(unsigned int order,
>  	struct kho_mem_phys_bits *bitmap = KHOSER_LOAD_PTR(elm->bitmap);
>  	unsigned long bit;
>  
> +	if (order > MAX_PAGE_ORDER) {

Preserved order can be larger than MAX_PAGE_ORDER. 
Let's make 'sz' unsigned long and add checks that calculations won't
overflow.

> +		pr_warn("invalid order %u for preserved bitmap\n", order);
> +		return;
> +	}
> +
>  	for_each_set_bit(bit, bitmap->preserve, PRESERVE_BITS) {
>  		int sz = 1 << (order + PAGE_SHIFT);
>  		phys_addr_t phys =
> -- 
> 2.53.0.335.g19a08e0c02-goog

-- 
Sincerely yours,
Mike.