[PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios

[PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios

[Hui Su]

migrate_device_range() and migrate_device_pfns() clear the entries
following a compound folio so that the PFN arrays retain their
page-granular representation.

If a compound folio extends beyond the end of the caller-provided range,
the loops clear all following folio entries without limiting them to the
number of slots remaining in the npages-sized array, causing an
out-of-bounds write.

Do not proceed with a compound folio if its page-granular representation
does not fit entirely in the remaining PFN array. If this happens, drop
any reference and lock acquired for the folio, clear the remaining
entries, and stop collecting.

Observed with a KASAN x86 QEMU kernel using the HMM
migrate_anon_huge_zero selftest. Closing /dev/hmm_dmirror0 after
migrating an anonymous huge page to device memory exercises:

  dmirror_fops_release()
    -> dmirror_device_evict_chunk()
      -> migrate_device_range()

Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Su <sh_def@163.com>
---
Changes in v2:
- Do not partially represent a compound folio when it does not fit in
  the remaining PFN array.
- Drop any reference and lock acquired for that folio, clear the
  remaining entries, and stop collecting, as suggested by Balbir Singh.

v1: https://lore.kernel.org/lkml/20260817074350.442493-2-sh_def@163.com/

 mm/migrate_device.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 908d2d4ec43a..69b8d0660bab 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1400,6 +1400,15 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
 
 		src_pfns[i] = migrate_device_pfn_lock(pfn);
 		nr = folio_nr_pages(folio);
+		if (nr > npages - i) {
+			if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
+				folio_unlock(folio);
+				folio_put(folio);
+			}
+			memset(&src_pfns[i], 0,
+			       (npages - i) * sizeof(*src_pfns));
+			break;
+		}
 		if (nr > 1) {
 			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
 			for (j = 1; j < nr; j++)
@@ -1434,6 +1443,15 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
 
 		src_pfns[i] = migrate_device_pfn_lock(src_pfns[i]);
 		nr = folio_nr_pages(folio);
+		if (nr > npages - i) {
+			if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
+				folio_unlock(folio);
+				folio_put(folio);
+			}
+			memset(&src_pfns[i], 0,
+			       (npages - i) * sizeof(*src_pfns));
+			break;
+		}
 		if (nr > 1) {
 			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
 			for (j = 1; j < nr; j++)
-- 
2.54.0

Re: [PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios

[Andrew Morton]

On Mon, 17 Aug 2026 20:08:00 +0800 Hui Su <sh_def@163.com> wrote:

> migrate_device_range() and migrate_device_pfns() clear the entries
> following a compound folio so that the PFN arrays retain their
> page-granular representation.
> 
> If a compound folio extends beyond the end of the caller-provided range,
> the loops clear all following folio entries without limiting them to the
> number of slots remaining in the npages-sized array, causing an
> out-of-bounds write.
> 
> Do not proceed with a compound folio if its page-granular representation
> does not fit entirely in the remaining PFN array. If this happens, drop
> any reference and lock acquired for the folio, clear the remaining
> entries, and stop collecting.
> 
> Observed with a KASAN x86 QEMU kernel using the HMM
> migrate_anon_huge_zero selftest. Closing /dev/hmm_dmirror0 after
> migrating an anonymous huge page to device memory exercises:
> 
>   dmirror_fops_release()
>     -> dmirror_device_evict_chunk()
>       -> migrate_device_range()

It isn't clear (to me, at least) what "exercises:" means.  Was there a
WARN?  Did the kernel crash?  Did it erase all my cat videos?

IOW, in detail, what are the userspace-visible effects of the bug? 
Please add this info to the changelog and maintain it.

Sashiko might have found some things.  Some pre-existing, some newly added:
	https://sashiko.dev/#/patchset/20260817120758.669807-3-sh_def@163.com

Re: [PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios

[Balbir Singh]

On 8/18/26 4:21 AM, Andrew Morton wrote:
> On Mon, 17 Aug 2026 20:08:00 +0800 Hui Su <sh_def@163.com> wrote:
> 
>> migrate_device_range() and migrate_device_pfns() clear the entries
>> following a compound folio so that the PFN arrays retain their
>> page-granular representation.
>>
>> If a compound folio extends beyond the end of the caller-provided range,
>> the loops clear all following folio entries without limiting them to the
>> number of slots remaining in the npages-sized array, causing an
>> out-of-bounds write.
>>
>> Do not proceed with a compound folio if its page-granular representation
>> does not fit entirely in the remaining PFN array. If this happens, drop
>> any reference and lock acquired for the folio, clear the remaining
>> entries, and stop collecting.
>>
>> Observed with a KASAN x86 QEMU kernel using the HMM
>> migrate_anon_huge_zero selftest. Closing /dev/hmm_dmirror0 after
>> migrating an anonymous huge page to device memory exercises:
>>
>>   dmirror_fops_release()
>>     -> dmirror_device_evict_chunk()
>>       -> migrate_device_range()
> 
> It isn't clear (to me, at least) what "exercises:" means.  Was there a
> WARN?  Did the kernel crash?  Did it erase all my cat videos?
> 
> IOW, in detail, what are the userspace-visible effects of the bug? 
> Please add this info to the changelog and maintain it.
> 
> Sashiko might have found some things.  Some pre-existing, some newly added:
> 	https://sashiko.dev/#/patchset/20260817120758.669807-3-sh_def@163.com
> 
> 

userspace-visible effects are a function of the usage of the API by the device
driver. If the device driver passes in npages that don't account for PMD pages,
it would be an issue. In the case above, closing the device will evict pages
from the device back to the CPU, but I am surprised that the size of npages was
not properly split, we should fix that as well.

FYI: I've tried reading through Sashiko's report, but there is too much
pre-existing content that it points to and it can be quite time consuming. I am
working on adding new test cases to try and catch some of the issues found

Balbir

Re: [PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios

[David Hildenbrand (Arm)]

On 8/17/26 14:08, Hui Su wrote:
> migrate_device_range() and migrate_device_pfns() clear the entries
> following a compound folio so that the PFN arrays retain their
> page-granular representation.
> 
> If a compound folio extends beyond the end of the caller-provided range,
> the loops clear all following folio entries without limiting them to the
> number of slots remaining in the npages-sized array, causing an
> out-of-bounds write.
> 
> Do not proceed with a compound folio if its page-granular representation
> does not fit entirely in the remaining PFN array. If this happens, drop
> any reference and lock acquired for the folio, clear the remaining
> entries, and stop collecting.
> 
> Observed with a KASAN x86 QEMU kernel using the HMM
> migrate_anon_huge_zero selftest. Closing /dev/hmm_dmirror0 after
> migrating an anonymous huge page to device memory exercises:
> 
>   dmirror_fops_release()
>     -> dmirror_device_evict_chunk()
>       -> migrate_device_range()
> 
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hui Su <sh_def@163.com>
> ---
> Changes in v2:
> - Do not partially represent a compound folio when it does not fit in
>   the remaining PFN array.
> - Drop any reference and lock acquired for that folio, clear the
>   remaining entries, and stop collecting, as suggested by Balbir Singh.
> 
> v1: https://lore.kernel.org/lkml/20260817074350.442493-2-sh_def@163.com/
> 
>  mm/migrate_device.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 908d2d4ec43a..69b8d0660bab 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -1400,6 +1400,15 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
>  
>  		src_pfns[i] = migrate_device_pfn_lock(pfn);
>  		nr = folio_nr_pages(folio);
> +		if (nr > npages - i) {
> +			if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
> +				folio_unlock(folio);
> +				folio_put(folio);
> +			}
> +			memset(&src_pfns[i], 0,
> +			       (npages - i) * sizeof(*src_pfns));
> +			break;
> +		}
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
>  			for (j = 1; j < nr; j++)
> @@ -1434,6 +1443,15 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
>  
>  		src_pfns[i] = migrate_device_pfn_lock(src_pfns[i]);
>  		nr = folio_nr_pages(folio);
> +		if (nr > npages - i) {
> +			if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
> +				folio_unlock(folio);
> +				folio_put(folio);
> +			}
> +			memset(&src_pfns[i], 0,
> +			       (npages - i) * sizeof(*src_pfns));
> +			break;
> +		}
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
>  			for (j = 1; j < nr; j++)

The code duplication here makes me angry. :) And using a memset on one branch but
not on the other is weird.

But is this the right fix or rather what

	https://lore.kernel.org/r/20260805231041.3791771-3-matthew.brost@intel.com 

tried? That fix would also need a cleanup but seems code-wise simpler. A cleanup could look like:

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 762c5cee8fecc..480bf59bc2425 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1419,10 +1419,9 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
        for (pfn = start, i = 0; i < npages; pfn++, i++) {
                struct page *page = pfn_to_page(pfn);
                struct folio *folio = page_folio(page);
-               unsigned int nr = 1;
+               unsigned int nr = min(folio_nr_pages(folio), npages - i);
 
                src_pfns[i] = migrate_device_pfn_lock(pfn);
-               nr = folio_nr_pages(folio);
                if (nr > 1) {
                        src_pfns[i] |= MIGRATE_PFN_COMPOUND;
                        for (j = 1; j < nr; j++)
@@ -1453,10 +1452,9 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
        for (i = 0; i < npages; i++) {
                struct page *page = pfn_to_page(src_pfns[i]);
                struct folio *folio = page_folio(page);
-               unsigned int nr = 1;
+               unsigned int nr = min(folio_nr_pages(folio), npages - i);
 
                src_pfns[i] = migrate_device_pfn_lock(src_pfns[i]);
-               nr = folio_nr_pages(folio);
                if (nr > 1) {
                        src_pfns[i] |= MIGRATE_PFN_COMPOUND;
                        for (j = 1; j < nr; j++)


And as a further cleanup, we'd better de-duplicate that code and possibly use a
memset for clearing, getting rid of j entirely.



-- 
Cheers,

David