[PATCH] mm: Take a page reference when removing device exclusive entries

[Alistair Popple <apopple@nvidia.com>]

Device exclusive page table entries are used to prevent CPU access to
a page whilst it is being accessed from a device. Typically this is
used to implement atomic operations when the underlying bus does not
support atomic access. When a CPU thread encounters a device exclusive
entry it locks the page and restores the original entry after calling
mmu notifiers to signal drivers that exclusive access is no longer
available.

The device exclusive entry holds a reference to the page making it
safe to access the struct page whilst the entry is present. However
the fault handling code does not hold the PTL when taking the page
lock. This means if there are multiple threads faulting concurrently
on the device exclusive entry one will remove the entry whilst others
will wait on the page lock without holding a reference.

This can lead to threads locking or waiting on a page with a zero
refcount. Whilst mmap_lock prevents the pages getting freed via
munmap() they may still be freed by a migration. This leads to
warnings such as PAGE_FLAGS_CHECK_AT_FREE due to the page being locked
when the refcount drops to zero. Note that during removal of the
device exclusive entry the PTE is currently re-checked under the PTL
so no futher bad page accesses occur once it is locked.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Cc: stable@vger.kernel.org
---
 mm/memory.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 8c8420934d60..b499bd283d8e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3623,8 +3623,19 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 	struct vm_area_struct *vma = vmf->vma;
 	struct mmu_notifier_range range;
 
-	if (!folio_lock_or_retry(folio, vma->vm_mm, vmf->flags))
+	/*
+	 * We need a page reference to lock the page because we don't
+	 * hold the PTL so a racing thread can remove the
+	 * device-exclusive entry and unmap the page. If the page is
+	 * free the entry must have been removed already.
+	 */
+	if (!get_page_unless_zero(vmf->page))
+		return 0;
+
+	if (!folio_lock_or_retry(folio, vma->vm_mm, vmf->flags)) {
+		put_page(vmf->page);
 		return VM_FAULT_RETRY;
+	}
 	mmu_notifier_range_init_owner(&range, MMU_NOTIFY_EXCLUSIVE, 0, vma,
 				vma->vm_mm, vmf->address & PAGE_MASK,
 				(vmf->address & PAGE_MASK) + PAGE_SIZE, NULL);
@@ -3637,6 +3648,7 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 
 	pte_unmap_unlock(vmf->pte, vmf->ptl);
 	folio_unlock(folio);
+	put_page(vmf->page);
 
 	mmu_notifier_invalidate_range_end(&range);
 	return 0;
-- 
2.39.2