[PATCH v2 08/17] kernel/events/uprobes: handle device-exclusive entries correctly in __replace_page()

[David Hildenbrand <david@redhat.com>]

Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
we can return with a device-exclusive entry from page_vma_mapped_walk().

__replace_page() is not prepared for that, so teach it about these
PFN swap PTEs. Note that device-private entries are so far not
applicable on that path, because GUP would never have returned such
folios (conversion to device-private happens by page migration, not
in-place conversion of the PTE).

There is a race between GUP and us locking the folio to look it up
using page_vma_mapped_walk(), so this is likely a fix (unless something
else could prevent that race, but it doesn't look like). pte_pfn() on
something that is not a present pte could give use garbage, and we'd
wrongly mess up the mapcount because it was already adjusted by calling
folio_remove_rmap_pte() when making the entry device-exclusive.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 kernel/events/uprobes.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2ca797cbe465f..cd6105b100325 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -173,6 +173,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	DEFINE_FOLIO_VMA_WALK(pvmw, old_folio, vma, addr, 0);
 	int err;
 	struct mmu_notifier_range range;
+	pte_t pte;
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr,
 				addr + PAGE_SIZE);
@@ -192,6 +193,16 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 	if (!page_vma_mapped_walk(&pvmw))
 		goto unlock;
 	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
+	pte = ptep_get(pvmw.pte);
+
+	/*
+	 * Handle PFN swap PTES, such as device-exclusive ones, that actually
+	 * map pages: simply trigger GUP again to fix it up.
+	 */
+	if (unlikely(!pte_present(pte))) {
+		page_vma_mapped_walk_done(&pvmw);
+		goto unlock;
+	}
 
 	if (new_page) {
 		folio_get(new_folio);
@@ -206,7 +217,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 		inc_mm_counter(mm, MM_ANONPAGES);
 	}
 
-	flush_cache_page(vma, addr, pte_pfn(ptep_get(pvmw.pte)));
+	flush_cache_page(vma, addr, pte_pfn(pte));
 	ptep_clear_flush(vma, addr, pvmw.pte);
 	if (new_page)
 		set_pte_at(mm, addr, pvmw.pte,
-- 
2.48.1