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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/1/23 10:10, 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>
> ---

LGTM. And I remember Vlastimil had the same concern before[1].
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

[1] 
https://lore.kernel.org/all/6b055821-ce14-4a6d-959c-25ade4a9bfd7@suse.cz/

>   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) &&
> +			    (blockpfn + (1UL << order) <= end_pfn)) {
>   				blockpfn += (1UL << order) - 1;
>   				page += (1UL << order) - 1;
>   				nr_scanned += (1UL << order) - 1;