linux-kernel - Re: [PATCH] mm: mmap: Fix general protection fault in unlink_file

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Message-Id: <20201008212304.e631c002648525a5ce54a102@linux-foundation.org>
Date:   Thu, 8 Oct 2020 21:23:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: mmap: Fix general protection fault in
 unlink_file_vma()

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

> Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Wed, 16 Sep 2020 05:07:33 -0400 Miaohe Lin <linmiaohe@...wei.com> 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" ;)