[PATCH v2 1/1] check that order of free pages falls within valid range

[Alexander Egorenkov]

Alexander Egorenkov <egorenar at linux.ibm.com> writes:

> This change makes __exclude_unnecessary_pages() more robust by
> verifying that the order of a free page is valid before computing the size
> of its memory block in the buddy system.
>
> The order of a free page cannot be larger than (MAX_ORDER - 1) because
> the array 'zone.free_area' is of size MAX_ORDER.
>
> This situation is reproducible with some s390x dumps:
>
> __exclude_unnecessary_pages: Invalid free page order: pfn=2690c0, order=52, max order=8
>
> References:
> - https://listman.redhat.com/archives/crash-utility/2021-September/009204.html
> - https://www.kernel.org/doc/gorman/html/understand/understand009.html
>
> Signed-off-by: Alexander Egorenkov <egorenar at linux.ibm.com>
> ---
>  makedumpfile.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/makedumpfile.c b/makedumpfile.c
> index 2ef345879524..56aa026e7b34 100644
> --- a/makedumpfile.c
> +++ b/makedumpfile.c
> @@ -6457,6 +6457,12 @@ __exclude_unnecessary_pages(unsigned long mem_map,
>  		else if ((info->dump_level & DL_EXCLUDE_FREE)
>  		    && info->page_is_buddy
>  		    && info->page_is_buddy(flags, _mapcount, private, _count)) {
> +			if (private >= ARRAY_LENGTH(zone.free_area)) {
> +				ERRMSG("Invalid free page order: pfn=%llx, order=%lu, max order=%lu\n",
> +				       pfn, private, ARRAY_LENGTH(zone.free_area) - 1);
> +				free(page_cache);
> +				return FALSE;
> +			}
>  			nr_pages = 1 << private;
>  			pfn_counter = &pfn_free;
>  		}
> -- 
> 2.34.1
>
>
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I found out when this can happen.

If e.g. a driver calls free_pages() and gives an order > max page order,
then __free_one_page() stores the given invalid page order in the
'private' member of struct page and gives it back to the buddy
allocator.

This is what actually happened in the dump i used to reproduce this issue
with makedumpfile.

Regards
Alex