Re: [PATCH] mm, KMSAN: Add missing shadow memory initialization in special allocation paths

[Muhammad Usama Anjum <usama.anjum@arm.com>]

On 31/03/2026 3:00 am, Ke Zhao wrote:
> 
> On 3/31/2026 4:39 AM, Usama Anjum wrote:
>> On 30/03/2026 9:36 am, Ke Zhao wrote:
>>> Some page allocation paths that call post_alloc_hook() but skip
>>> kmsan_alloc_page(), leaving stale KMSAN shadow on allocated pages.
>>> Fix this by explicitly calling kmsan_alloc_page() after they
>>> successfully get new pages.
>>>
>>> Reported-by:syzbot+2aee6839a252e612ce34@syzkaller.appspotmail.com
>>> Closes:https://syzkaller.appspot.com/bug?extid=2aee6839a252e612ce34
>>>
>>> Signed-off-by: Ke Zhao<ke.zhao.kernel@gmail.com>
>>> ---
>>>   mm/page_alloc.c | 13 +++++++++++++
>>>   1 file changed, 13 insertions(+)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 2d4b6f1a554e..6435e8708ef4 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5189,6 +5189,10 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
>>>   
>>>   		prep_new_page(page, 0, gfp, 0);
>>>   		set_page_refcounted(page);
>>> +
>>> +		trace_mm_page_alloc(page, 0, gfp, ac.migratetype);
>>> +		kmsan_alloc_page(page, 0, gfp);
>>> +
>>>   		page_array[nr_populated++] = page;
>>>   	}
>>>   
>>> @@ -6911,6 +6915,12 @@ static void split_free_frozen_pages(struct list_head *list, gfp_t gfp_mask)
>>>   			int i;
>>>   
>>>   			post_alloc_hook(page, order, gfp_mask);
>>> +			/*
>>> +			 * Initialize KMSAN state right after post_alloc_hook().
>>> +			 * This prepares the pages for subsequent outer callers
>>> +			 * that might free sub-pages after the split.
>>> +			 */
>>> +			kmsan_alloc_page(page, order, gfp_mask);
>>>   			if (!order)
>>>   				continue;
>>>   
>>> @@ -7117,6 +7127,9 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
>>>   
>>>   		check_new_pages(head, order);
>>>   		prep_new_page(head, order, gfp_mask, 0);
>>> +
>>> +		trace_mm_page_alloc(page, order, gfp_mask, get_pageblock_migratetype(page));
>>> +		kmsan_alloc_page(page, order, gfp_mask);
>> There is no page defined in this function. Most probably you wanted
>> to use head in place of page here.
>>
>> How did you compiled and tested this change?
> Sorry, I just simply compiled with the change but add wrong code into 
> the commit. I can hardly make an environment that could trigger the same 
> warning here. I'm not sure if I can trigger syzbot to test this.
Please try to send another version (v2) of this patch and then
[1] follow instructions mentioned here.

[1] https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches

>>>   	} else {
>>>   		ret = -EINVAL;
>>>   		WARN(true, "PFN range: requested [%lu, %lu), allocated [%lu, %lu)\n",
>>>
>>> ---
>>> base-commit: bbeb83d3182abe0d245318e274e8531e5dd7a948
>>> change-id: 20260325-fix-kmsan-e291f752a949
>>>
>>> Best regards,
>> Thanks,
>> Usama