[PATCH v2 1/3] mmap: Fix vma_iterator in error path of vma_merge()

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

When merging of the previous VMA fails after the vma iterator has been
moved to the previous entry, the vma iterator must be advanced to ensure
the caller takes the correct action on the next vma iterator event.  Fix
this by adding a vma_next() call to the error path.

Users would not notice the effects as it would result in an extra
vma_merge() call if the first call ended due to an out-of-memory event.

Link: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
Closes: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
Fixes: 18b098af2890 ("vma_merge: set vma iterator to correct position.")
Cc: stable@vger.kernel.org
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
---
 mm/mmap.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index b56a7f0c9f85..a4eb5a5626bb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -975,7 +975,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 
 	/* Error in anon_vma clone. */
 	if (err)
-		return NULL;
+		goto anon_vma_fail;
 
 	if (vma_start < vma->vm_start || vma_end > vma->vm_end)
 		vma_expanded = true;
@@ -988,7 +988,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	}
 
 	if (vma_iter_prealloc(vmi, vma))
-		return NULL;
+		goto prealloc_fail;
 
 	init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
 	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
@@ -1016,6 +1016,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	vma_complete(&vp, vmi, mm);
 	khugepaged_enter_vma(res, vm_flags);
 	return res;
+
+prealloc_fail:
+anon_vma_fail:
+	if (merge_prev)
+		vma_next(vmi);
+	return NULL;
 }
 
 /*
-- 
2.40.1