mm: mmap: Fix general protection fault in unlink_file_vma()

The syzbot reported the below general protection fault:

general protection fault, probably for non-canonical address
0xe00eeaee0000003b: 0000 [#1] PREEMPT SMP KASAN
KASAN: maybe wild-memory-access in range [0x00777770000001d8-0x00777770000001df]
CPU: 1 PID: 10488 Comm: syz-executor721 Not tainted 5.9.0-rc3-syzkaller #0

It's because the ->mmap() callback can change vma->vm_file and fput the original file. But the commit d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible") failed to catch this case and always fput() the original file, hence add an extra fput().

[ Thanks Hillf for pointing this extra fput() out. ]

(quoted diff) +5 -2

@@ -1815,7 +1815,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 			merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
 				NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
 			if (merge) {
-				fput(file);
+				/* ->mmap() can change vma->vm_file and fput the original file. So
+				 * fput the vma->vm_file here or we would add an extra fput for file
+				 * and cause general protection fault ultimately.
+				 */
+				fput(vma->vm_file);
 				vm_area_free(vma);
 				vma = merge;
 				/* Update vm_flags and possible addr to pick up the change. We don't
--

2.19.1