[PATCH] mm/vma: read anon_vma carefully in validate_mm()

[Lorenzo Stoakes (ARM)]

Commit 38564710f443 ("mm: implement and use vma_has_anon_rmap(), silence
KCSAN") rather clumsily converted the read of vma->anon_vma into a
vma_has_anon_rmap() without accounting for the separation between
assignment of anon_vma and later use of it.

vma_has_anon_rmap() is intended as an optimistic check, so doesn't need to
protect against a torn read in vma->anon_vma. The caller is expected to
take care of this themselves if they then in fact access the field.

validate_mm() does indeed then reference the field, so resolve the issue
via READ_ONCE(), and simplify the code by separating this into a new
function validate_mm_anon_rmap().

Finally, add a vma_assert_stabilised() to make it clear the required state
of the VMA.

Fixes: 38564710f443 ("mm: implement and use vma_has_anon_rmap(), silence KCSAN")
Reported-by: syzbot+10e72ea5747737bcd2ad@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6ae78418.6552420f.2a0efc.005c.GAE@xxxxxxxxxx/
Reported-by: Songsen Yang <yangsongsen@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/20261102074414.10902-1-yangsongsen@xxxxxxxxxxxx/
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/vma.c | 36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index d416ba58eb4c..9f02f19a7845 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -669,6 +669,28 @@ static int dup_anon_vma(struct vm_area_struct *dst,
 }
 
 #ifdef CONFIG_DEBUG_VM_MAPLE_TREE
+#ifdef CONFIG_DEBUG_VM_RB
+static void validate_mm_anon_rmap(struct vm_area_struct *vma)
+{
+	struct anon_vma *anon_vma = READ_ONCE(vma->anon_vma);
+	struct anon_vma_chain *avc;
+
+	vma_assert_stabilised(vma);
+
+	if (!anon_vma)
+		return;
+
+	anon_vma_lock_read(anon_vma);
+	list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
+		anon_rmap_tree_verify(avc);
+	anon_vma_unlock_read(anon_vma);
+}
+#else
+static void validate_mm_anon_rmap(struct vm_area_struct *vma)
+{
+}
+#endif
+
 void validate_mm(struct mm_struct *mm)
 {
 	int bug = 0;
@@ -678,10 +700,6 @@ void validate_mm(struct mm_struct *mm)
 
 	mt_validate(&mm->mm_mt);
 	for_each_vma(vmi, vma) {
-#ifdef CONFIG_DEBUG_VM_RB
-		struct anon_vma *anon_vma = vma->anon_vma;
-		struct anon_vma_chain *avc;
-#endif
 		unsigned long vmi_start, vmi_end;
 		bool warn = 0;
 
@@ -702,14 +720,8 @@ void validate_mm(struct mm_struct *mm)
 			vma_iter_dump_tree(&vmi);
 		}
 
-#ifdef CONFIG_DEBUG_VM_RB
-		if (vma_has_anon_rmap(vma)) {
-			anon_vma_lock_read(anon_vma);
-			list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
-				anon_rmap_tree_verify(avc);
-			anon_vma_unlock_read(anon_vma);
-		}
-#endif
+		validate_mm_anon_rmap(vma);
+
 		/* Check for a infinite loop */
 		if (++i > mm->map_count + 10) {
 			i = -1;

---
base-commit: b61fa14ce6bcbba2cb9a96feea31b760226d6b43
change-id: 20261102-fix-validate_mm-b7930d41388b

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>