[PATCH v3 2/3] mmap: Fix error paths with dup_anon_vma()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

When the calling function fails after the dup_anon_vma(), the
duplication of the anon_vma is not being undone.  Add the necessary
unlink_anon_vma() call to the error paths that are missing them.

This issue showed up during inspection of the error path in vma_merge()
for an unrelated vma iterator issue.

Users may experience increased memory usage, which may be problematic as
the failure would likely be caused by a low memory situation.

Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
Cc: stable@vger.kernel.org
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
---
 mm/mmap.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index acb7dea49e23..f9f0a5fe4db4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -583,11 +583,12 @@ static inline void vma_complete(struct vma_prepare *vp,
  * dup_anon_vma() - Helper function to duplicate anon_vma
  * @dst: The destination VMA
  * @src: The source VMA
+ * @dup: Pointer to the destination VMA when successful.
  *
  * Returns: 0 on success.
  */
 static inline int dup_anon_vma(struct vm_area_struct *dst,
-			       struct vm_area_struct *src)
+		struct vm_area_struct *src, struct vm_area_struct **dup)
 {
 	/*
 	 * Easily overlooked: when mprotect shifts the boundary, make sure the
@@ -595,9 +596,15 @@ static inline int dup_anon_vma(struct vm_area_struct *dst,
 	 * anon pages imported.
 	 */
 	if (src->anon_vma && !dst->anon_vma) {
+		int ret;
+
 		vma_assert_write_locked(dst);
 		dst->anon_vma = src->anon_vma;
-		return anon_vma_clone(dst, src);
+		ret = anon_vma_clone(dst, src);
+		if (ret)
+			return ret;
+
+		*dup = dst;
 	}
 
 	return 0;
@@ -624,6 +631,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	       unsigned long start, unsigned long end, pgoff_t pgoff,
 	       struct vm_area_struct *next)
 {
+	struct vm_area_struct *anon_dup = NULL;
 	bool remove_next = false;
 	struct vma_prepare vp;
 
@@ -633,7 +641,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 		remove_next = true;
 		vma_start_write(next);
-		ret = dup_anon_vma(vma, next);
+		ret = dup_anon_vma(vma, next, &anon_dup);
 		if (ret)
 			return ret;
 	}
@@ -661,6 +669,8 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	return 0;
 
 nomem:
+	if (anon_dup)
+		unlink_anon_vmas(anon_dup);
 	return -ENOMEM;
 }
 
@@ -860,6 +870,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 {
 	struct vm_area_struct *curr, *next, *res;
 	struct vm_area_struct *vma, *adjust, *remove, *remove2;
+	struct vm_area_struct *anon_dup = NULL;
 	struct vma_prepare vp;
 	pgoff_t vma_pgoff;
 	int err = 0;
@@ -927,18 +938,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 		vma_start_write(next);
 		remove = next;				/* case 1 */
 		vma_end = next->vm_end;
-		err = dup_anon_vma(prev, next);
+		err = dup_anon_vma(prev, next, &anon_dup);
 		if (curr) {				/* case 6 */
 			vma_start_write(curr);
 			remove = curr;
 			remove2 = next;
 			if (!next->anon_vma)
-				err = dup_anon_vma(prev, curr);
+				err = dup_anon_vma(prev, curr, &anon_dup);
 		}
 	} else if (merge_prev) {			/* case 2 */
 		if (curr) {
 			vma_start_write(curr);
-			err = dup_anon_vma(prev, curr);
+			err = dup_anon_vma(prev, curr, &anon_dup);
 			if (end == curr->vm_end) {	/* case 7 */
 				remove = curr;
 			} else {			/* case 5 */
@@ -954,7 +965,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			vma_end = addr;
 			adjust = next;
 			adj_start = -(prev->vm_end - addr);
-			err = dup_anon_vma(next, prev);
+			err = dup_anon_vma(next, prev, &anon_dup);
 		} else {
 			/*
 			 * Note that cases 3 and 8 are the ONLY ones where prev
@@ -968,7 +979,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 				vma_pgoff = curr->vm_pgoff;
 				vma_start_write(curr);
 				remove = curr;
-				err = dup_anon_vma(next, curr);
+				err = dup_anon_vma(next, curr, &anon_dup);
 			}
 		}
 	}
@@ -1018,6 +1029,9 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	return res;
 
 prealloc_fail:
+	if (anon_dup)
+		unlink_anon_vmas(anon_dup);
+
 anon_vma_fail:
 	vma_iter_set(vmi, addr);
 	vma_iter_load(vmi);
-- 
2.40.1