[PATCH 1/3] lib: test_hmm: evict device pages on file close to avoid use-after-free

Alistair Popple
2026-03-31T06:35:07Z

When dmirror_fops_release() is called it frees the dmirror struct but
doesn't migrate device private pages back to system memory first. This
leaves those pages with a dangling zone_device_data pointer to the freed
dmirror.

If a subsequent fault occurs on those pages (eg. during coredump) the
dmirror_devmem_fault() callback dereferences the stale pointer causing a
kernel panic. This was reported [1] when running mm/ksft_hmm.sh on
arm64, where a test failure triggered SIGABRT and the resulting coredump
walked the VMAs faulting in the stale device private pages.

Fix this by calling dmirror_device_evict_chunk() for each devmem chunk
in dmirror_fops_release() to migrate all device private pages back to
system memory before freeing the dmirror struct. The function is moved
earlier in the file to avoid a forward declaration.

Fixes: b2ef9f5a5cb3 ("mm/hmm/test: add selftest driver for HMM")
Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
Closes: https://lore.kernel.org/linux-mm/8bd0396a-8997-4d2e-a13f-5aac033083d7@linux.dev/
Signed-off-by: Alistair Popple <apopple@nvidia.com>

---

Note that I wasn't able to replicate the exact crash in [1] although I
replicated something similar. So I haven't been able to verify this
fixes the crash conclusively, but it should.

[1] https://lore.kernel.org/linux-mm/8bd0396a-8997-4d2e-a13f-5aac033083d7@linux.dev/
---
lib/test_hmm.c | 112 +++++++++++++++++++++++++++----------------------
1 file changed, 62 insertions(+), 50 deletions(-)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 0964d53365e6..79fe7d233df1 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -185,11 +185,73 @@ static int dmirror_fops_open(struct inode *inode, struct file *filp)
return 0;
}

+static void dmirror_device_evict_chunk(struct dmirror_chunk *chunk)
+{
+ unsigned long start_pfn = chunk->pagemap.range.start >> PAGE_SHIFT;
+ unsigned long end_pfn = chunk->pagemap.range.end >> PAGE_SHIFT;
+ unsigned long npages = end_pfn - start_pfn + 1;
+ unsigned long i;
+ unsigned long *src_pfns;
+ unsigned long *dst_pfns;
+ unsigned int order = 0;
+
+ src_pfns = kvcalloc(npages, sizeof(*src_pfns), GFP_KERNEL | __GFP_NOFAIL);
+ dst_pfns = kvcalloc(npages, sizeof(*dst_pfns), GFP_KERNEL | __GFP_NOFAIL);
+
+ migrate_device_range(src_pfns, start_pfn, npages);
+ for (i = 0; i < npages; i++) {
+ struct page *dpage, *spage;
+
+ spage = migrate_pfn_to_page(src_pfns[i]);
+ if (!spage || !(src_pfns[i] & MIGRATE_PFN_MIGRATE))
+ continue;
+
+ if (WARN_ON(!is_device_private_page(spage) &&
+ !is_device_coherent_page(spage)))
+ continue;
+
+ order = folio_order(page_folio(spage));
+ spage = BACKING_PAGE(spage);
+ if (src_pfns[i] & MIGRATE_PFN_COMPOUND) {
+ dpage = folio_page(folio_alloc(GFP_HIGHUSER_MOVABLE,
+ order), 0);
+ } else {
+ dpage = alloc_page(GFP_HIGHUSER_MOVABLE | __GFP_NOFAIL);
+ order = 0;
+ }
+
+ /* TODO Support splitting here */
+ lock_page(dpage);
+ dst_pfns[i] = migrate_pfn(page_to_pfn(dpage));
+ if (src_pfns[i] & MIGRATE_PFN_WRITE)
+ dst_pfns[i] |= MIGRATE_PFN_WRITE;
+ if (order)
+ dst_pfns[i] |= MIGRATE_PFN_COMPOUND;
+ folio_copy(page_folio(dpage), page_folio(spage));
+ }
+ migrate_device_pages(src_pfns, dst_pfns, npages);
+ migrate_device_finalize(src_pfns, dst_pfns, npages);
+ kvfree(src_pfns);
+ kvfree(dst_pfns);
+}
+
static int dmirror_fops_release(struct inode *inode, struct file *filp)
{
struct dmirror *dmirror = filp->private_data;
+ struct dmirror_device *mdevice = dmirror->mdevice;
+ int i;

mmu_interval_notifier_remove(&dmirror->notifier);
+
+ if (mdevice->devmem_chunks) {
+ for (i = 0; i < mdevice->devmem_count; i++) {
+ struct dmirror_chunk *devmem =
+ mdevice->devmem_chunks[i];
+
+ dmirror_device_evict_chunk(devmem);
+ }
+ }
+
xa_destroy(&dmirror->pt);
kfree(dmirror);
return 0;
@@ -1377,56 +1439,6 @@ static int dmirror_snapshot(struct dmirror *dmirror,
return ret;
}

-static void dmirror_device_evict_chunk(struct dmirror_chunk *chunk)
-{
- unsigned long start_pfn = chunk->pagemap.range.start >> PAGE_SHIFT;
- unsigned long end_pfn = chunk->pagemap.range.end >> PAGE_SHIFT;
- unsigned long npages = end_pfn - start_pfn + 1;
- unsigned long i;
- unsigned long *src_pfns;
- unsigned long *dst_pfns;
- unsigned int order = 0;
-
- src_pfns = kvcalloc(npages, sizeof(*src_pfns), GFP_KERNEL | __GFP_NOFAIL);
- dst_pfns = kvcalloc(npages, sizeof(*dst_pfns), GFP_KERNEL | __GFP_NOFAIL);
-
- migrate_device_range(src_pfns, start_pfn, npages);
- for (i = 0; i < npages; i++) {
- struct page *dpage, *spage;
-
- spage = migrate_pfn_to_page(src_pfns[i]);
- if (!spage || !(src_pfns[i] & MIGRATE_PFN_MIGRATE))
- continue;
-
- if (WARN_ON(!is_device_private_page(spage) &&
- !is_device_coherent_page(spage)))
- continue;
-
- order = folio_order(page_folio(spage));
- spage = BACKING_PAGE(spage);
- if (src_pfns[i] & MIGRATE_PFN_COMPOUND) {
- dpage = folio_page(folio_alloc(GFP_HIGHUSER_MOVABLE,
- order), 0);
- } else {
- dpage = alloc_page(GFP_HIGHUSER_MOVABLE | __GFP_NOFAIL);
- order = 0;
- }
-
- /* TODO Support splitting here */
- lock_page(dpage);
- dst_pfns[i] = migrate_pfn(page_to_pfn(dpage));
- if (src_pfns[i] & MIGRATE_PFN_WRITE)
- dst_pfns[i] |= MIGRATE_PFN_WRITE;
- if (order)
- dst_pfns[i] |= MIGRATE_PFN_COMPOUND;
- folio_copy(page_folio(dpage), page_folio(spage));
- }
- migrate_device_pages(src_pfns, dst_pfns, npages);
- migrate_device_finalize(src_pfns, dst_pfns, npages);
- kvfree(src_pfns);
- kvfree(dst_pfns);
-}
-
/* Removes free pages from the free list so they can't be re-allocated */
static void dmirror_remove_free_pages(struct dmirror_chunk *devmem)
{
--
2.53.0