[PATCH v1] mm: kmsan: Fix poisoning of high-order non-compound pages

[Ryan Roberts]

kmsan_free_page() is called by the page allocator's free_pages_prepare()
during page freeing. It's job is to poison all the memory covered by the
page. It can be called with an order-0 page, a compound high-order page
or a non-compound high-order page. But page_size() only works for
order-0 and compound pages. For a non-compound high-order page it will
incorrectly return PAGE_SIZE.

The implication is that the tail pages of a high-order non-compound page
do not get poisoned at free, so any invalid access while they are free
could go unnoticed. It looks like the pages will be poisoned again at
allocaiton time, so that would bookend the window.

Fix this by using the order parameter to calculate the size.

Fixes: b073d7f8aee4 ("mm: kmsan: maintain KMSAN metadata for page operations")
Cc: sta...@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.r...@arm.com>
---

Hi,

I noticed this during code review, so perhaps I've just misunderstood the intent
of the code.

I don't have the means to compile and run on x86 with KMSAN enabled though, so
punting this out hoping someone might be able to validate/test. I guess there is
a small chance this could lead to KMSAN finding some new issues?

Applies against today's mm-unstable (344d3580dacd).

Thanks,
Ryan


mm/kmsan/shadow.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/kmsan/shadow.c b/mm/kmsan/shadow.c
index e7f554a31bb4..9e1c5f2b7a41 100644
--- a/mm/kmsan/shadow.c
+++ b/mm/kmsan/shadow.c
@@ -207,7 +207,7 @@ void kmsan_free_page(struct page *page, unsigned int order)
if (!kmsan_enabled || kmsan_in_runtime())
return;
kmsan_enter_runtime();
- kmsan_internal_poison_memory(page_address(page), page_size(page),
+ kmsan_internal_poison_memory(page_address(page), PAGE_SIZE << order,
GFP_KERNEL & ~(__GFP_RECLAIM),
KMSAN_POISON_CHECK | KMSAN_POISON_FREE);
kmsan_leave_runtime();
--
2.43.0

[Andrew Morton]

On Sun, 4 Jan 2026 13:43:47 +0000 Ryan Roberts <ryan.r...@arm.com> wrote:

> kmsan_free_page() is called by the page allocator's free_pages_prepare()
> during page freeing. It's job is to poison all the memory covered by the
> page. It can be called with an order-0 page, a compound high-order page
> or a non-compound high-order page. But page_size() only works for
> order-0 and compound pages. For a non-compound high-order page it will
> incorrectly return PAGE_SIZE.
>
> The implication is that the tail pages of a high-order non-compound page
> do not get poisoned at free, so any invalid access while they are free
> could go unnoticed. It looks like the pages will be poisoned again at
> allocaiton time, so that would bookend the window.
>
> Fix this by using the order parameter to calculate the size.
>
> Fixes: b073d7f8aee4 ("mm: kmsan: maintain KMSAN metadata for page operations")
> Cc: sta...@vger.kernel.org
> Signed-off-by: Ryan Roberts <ryan.r...@arm.com>
> ---
>
> Hi,
>
> I noticed this during code review, so perhaps I've just misunderstood the intent
> of the code.
>
> I don't have the means to compile and run on x86 with KMSAN enabled though, so
> punting this out hoping someone might be able to validate/test. I guess there is
> a small chance this could lead to KMSAN finding some new issues?

We'll see, I'll park this in mm-new to get it a little testing, see if
anything is shaken out. If all looks good and if the KMSAN maintainers
are OK with it I'll later move the patch into mm-hotfixes for more
expedited upstreaming.

[Alexander Potapenko]

On Sun, Jan 4, 2026 at 7:02 PM Andrew Morton <ak...@linux-foundation.org> wrote:
>
> On Sun, 4 Jan 2026 13:43:47 +0000 Ryan Roberts <ryan.r...@arm.com> wrote:
>
> > kmsan_free_page() is called by the page allocator's free_pages_prepare()
> > during page freeing. It's job is to poison all the memory covered by the
> > page. It can be called with an order-0 page, a compound high-order page
> > or a non-compound high-order page. But page_size() only works for
> > order-0 and compound pages. For a non-compound high-order page it will
> > incorrectly return PAGE_SIZE.
> >
> > The implication is that the tail pages of a high-order non-compound page
> > do not get poisoned at free, so any invalid access while they are free
> > could go unnoticed. It looks like the pages will be poisoned again at
> > allocaiton time, so that would bookend the window.
> >
> > Fix this by using the order parameter to calculate the size.
> >
> > Fixes: b073d7f8aee4 ("mm: kmsan: maintain KMSAN metadata for page operations")
> > Cc: sta...@vger.kernel.org
> > Signed-off-by: Ryan Roberts <ryan.r...@arm.com>

Reviewed-by: Alexander Potapenko <gli...@google.com>
Tested-by: Alexander Potapenko <gli...@google.com>

Thanks!
I'll send out a follow-up patch with a test for this behavior.