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
RIP: 0010:unlink_file_vma+0x57/0xb0 mm/mmap.c:164
Call Trace:
free_pgtables+0x1b3/0x2f0 mm/memory.c:415
exit_mmap+0x2c0/0x530 mm/mmap.c:3184
__mmput+0x122/0x470 kernel/fork.c:1076
mmput+0x53/0x60 kernel/fork.c:1097
exit_mm kernel/exit.c:483 [inline]
do_exit+0xa8b/0x29f0 kernel/exit.c:793
do_group_exit+0x125/0x310 kernel/exit.c:903
get_signal+0x428/0x1f00 kernel/signal.c:2757
arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
exit_to_user_mode_prepare+0x1ae/0x200 kernel/entry/common.c:167
syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:242
entry_SYSCALL_64_after_hwframe+0x44/0xa9

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. ]

Fixes: d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible")
Reported-by: [e-mail]
Signed-off-by: Miaohe Lin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Hongxiang Lou <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: John Hubbard <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

mm/mmap.c

···
 			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