mm: correctly reference merged VMA

On second merge attempt on mmap() we incorrectly discard the possibly
merged VMA, resulting in a possible use-after-free (and most certainly a
reference to the wrong VMA) in this instance in the subsequent
__mmap_complete() invocation.

Correct this mistake by reassigning vma correctly if a merge succeeds in
this case.

Link: https://lkml.kernel.org/r/20241206215229.244413-1-lorenzo.stoakes@oracle.com
Fixes: 5ac87a8 ("mm: defer second attempt at merge on mmap()")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Suggested-by: Jann Horn <jannh@google.com>
Reported-by: syzbot+91cf8da9401355f946c3@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67536a25.050a0220.a30f1.0149.GAE@google.com/
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vma.c

@@ -2460,10 +2460,13 @@ unsigned long __mmap_region(struct file *file, unsigned long addr,
 
 	/* If flags changed, we might be able to merge, so try again. */
 	if (map.retry_merge) {
+		struct vm_area_struct *merged;
 		VMG_MMAP_STATE(vmg, &map, vma);
 
 		vma_iter_config(map.vmi, map.addr, map.end);
-		vma_merge_existing_range(&vmg);
+		merged = vma_merge_existing_range(&vmg);
+		if (merged)
+			vma = merged;
 	}
 
 	__mmap_complete(&map, vma);