Re: [PATCH] mm: mmap: Fix general protection fault in unlink_file_vma()

[Andrew Morton]

On Thu, 8 Oct 2020 07:17:18 +0000 linmiaohe <linmiaohe@xxxxxxxxxx> wrote:

> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, 16 Sep 2020 05:07:33 -0400 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
> >
> >> 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
> >> Trace:
> >>  free_pgtables+0x1b3/0x2f0 mm/memory.c:415
> >>  exit_mmap+0x2c0/0x530 mm/mmap.c:3184
> >>  __mmput+0x122/0x470 kernel/fork.c:1076
> >> 
> >> 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().
> >> 
>
> ...
>
> >
> >is this using the correct file?  I think it is, but please do check.
> >
> 
> Many thanks for your reply.
> 
> Yes, I think so too. We do deny_write_access and mapping_map_writable on @file, so we should undo all of this on @file.
> Since @file is unchanged over the second vma_merge() time, we'are using the correct @file to undo our temporary denial count.
> 
> But how should I check this explicitly ? I can't find out a way to do this. Could you please figure it out for me?

I meant "please check (review) the code as it now is", not "please add
a check" ;)