Re: [PATCH] nilfs2: Fix nilfs_sufile_mark_dirty() not set segment usage as dirty

[Ryusuke Konishi]

On Sat, Nov 19, 2022 at 4:17 PM Ryusuke Konishi wrote:
>
> 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@xxxxxxxxxx> 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);

Sorry, the location of this down_write() was wrong in this email.
In my tested change, I put it before
nilfs_sufile_get_segment_usage_block() like others.

> +       down_write(&NILFS_MDT(sufile)->mi_sem);
> >       ret = nilfs_sufile_get_segment_usage_block(sufile, segnum, 0, &bh);
> >       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.


Regards,
Ryusuke Konishi


>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> 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