Re: [PATCH] mm/compaction: fix UBSAN shift-out-of-bounds warning

[David Hildenbrand <david@redhat.com>]

On 23.01.25 03:10, Liu Shixin wrote:
> syzkaller reported a UBSAN shift-out-of-bounds warning of (1UL << order)
> in isolate_freepages_block(). The bogus compound_order can be any value
> because it is union with flags. Add back the MAX_PAGE_ORDER check to fix
> the warning.
> 
> Fixes: 3da0272a4c7d ("mm/compaction: correctly return failure with bogus compound_order in strict mode")
> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> ---
>   mm/compaction.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index a2b16b08cbbff..384e4672998e5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -630,7 +630,8 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>   		if (PageCompound(page)) {
>   			const unsigned int order = compound_order(page);
>   
> -			if (blockpfn + (1UL << order) <= end_pfn) {
> +			if ((order <= MAX_PAGE_ORDER) &&

hugetlb can exceed MAX_PAGE_ORDER, but end_pfn cannot. So I think what 
you have here will not change the exiting handling.

Acked-by: David Hildenbrand <david@redhat.com>


Note: the whole "-1" with stride != 1 looks odd ...

-- 
Cheers,

David / dhildenb