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From:   Ryusuke Konishi <konishi.ryusuke@gmail.com>
Date:   Sat, 19 Nov 2022 16:17:46 +0900
Message-ID: <CAKFNMom=vjGrXJoc02ut8GocQ6hMmHrkcdReEvk-ykcE4p0b-w@mail.gmail.com>
Subject: Re: [PATCH] nilfs2: Fix nilfs_sufile_mark_dirty() not set segment
 usage as dirty
To:     Chen Zhongjin <chenzhongjin@huawei.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org
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Hi Chen Zhongjin,

On Sat, Nov 19, 2022 at 2:29 PM Chen Zhongjin wrote:
>
>
> On 2022/11/19 13:24, Chen Zhongjin wrote:
> > On 2022/11/19 6:11, Andrew Morton wrote:
> >> On Fri, 18 Nov 2022 14:33:04 +0800 Chen Zhongjin
> >> <chenzhongjin@huawei.com> wrote:
> >>
> >>> In nilfs_sufile_mark_dirty(), the buffer and inode are set dirty, but
> >>> nilfs_segment_usage is not set dirty, which makes it can be found by
> >>> nilfs_sufile_alloc() because it checks nilfs_segment_usage_clean(su).
> >>>
> >>> This will cause the problem reported by syzkaller:
> >>> https://syzkaller.appspot.com/bug?id=c7c4748e11ffcc367cef04f76e02e931833cbd24
> >>>
> >>>
> >>> It's because the case starts with segbuf1.segnum = 3, nextnum = 4, and
> >>> nilfs_sufile_alloc() not called to allocate a new segment.
> >>>
> >>> The first time nilfs_segctor_extend_segments() allocated segment
> >>> segbuf2.segnum = segbuf1.nextnum = 4, then nilfs_sufile_alloc() found
> >>> nextnextnum = 4 segment because its su is not set dirty.
> >>> So segbuf2.nextnum = 4, which causes next segbuf3.segnum = 4.
> >>>
> >>> sb_getblk() will get same bh for segbuf2 and segbuf3, and this bh is
> >>> added to both buffer lists of two segbuf.
> >>> It makes the list head of second list linked to the first one. When
> >>> iterating the first one, it will access and deref the head of second,
> >>> which causes NULL pointer dereference.
> >>>
> >>> Fixes: 9ff05123e3bf ("nilfs2: segment constructor")
> >> Merged in 2009!
> >
> > Yes, seems it is introduced at the beginning of this file and the
> > function called nilfs_touch_segusage().
> >

Could you please resubmit the patch reflecting the following comments ?

After I replied to Andrew, I noticed them.
Also, When reposting, it would be helpful if you could add all the
tags I asked for Andrew in advance.

Comments:
1) Please change nilfs_sufile_mark_dirty() so that it protects the
segusage modification
with &NILFS_MDT(sufile)->mi_sem:

> --- a/fs/nilfs2/sufile.c
> +++ b/fs/nilfs2/sufile.c
> @@ -495,12 +495,18 @@ void nilfs_sufile_do_free(struct inode *sufile, __u64 segnum,
>  int nilfs_sufile_mark_dirty(struct inode *sufile, __u64 segnum)
>  {
>       struct buffer_head *bh;
> +     void *kaddr;
> +     struct nilfs_segment_usage *su;
>       int ret;
>
>       ret = nilfs_sufile_get_segment_usage_block(sufile, segnum, 0, &bh);

+       down_write(&NILFS_MDT(sufile)->mi_sem);
>       if (!ret) {
>               mark_buffer_dirty(bh);
>               nilfs_mdt_mark_dirty(sufile);
> +             kaddr = kmap_atomic(bh->b_page);
> +             su = nilfs_sufile_block_get_segment_usage(sufile, segnum, bh, kaddr);
> +             nilfs_segment_usage_set_dirty(su);
> +             kunmap_atomic(kaddr);
>               brelse(bh);
>       }
+       up_write(&NILFS_MDT(sufile)->mi_sem);
>       return ret;

All functions that modify metadata on the sufile need protection with
this R/W semaphore.
You may not see this protection for some sufile functions as is, but
in that case, the wrapper function that uses them acquires this R/W
semaphore instead.

Since I retested for this change as well, you don't have to drop my
"Tested-by" tag.

2) Please use the following complete email address for the
"Reported-by" tag of syzbot.

Reported-by: syzbot+77e4f005cb899d4268d1@syzkaller.appspotmail.com

Your tag is partially abbreviated.  I don't know that abbreviation is
valid, but there are very few examples of such.
And even if it's valid for syzbot, I don't think that omission is
desirable as some tools may not support it.

Thanks,
Ryusuke Konishi