Re: [PATCH v2] nilfs2: Fix nilfs_sufile_mark_dirty() not set segment usage as dirty

[Chen Zhongjin]

Hi,

On 2022/11/19 22:09, Ryusuke Konishi wrote:
> On Sat, Nov 19, 2022 at 6:37 PM Chen Zhongjin 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).
> The body of the patch looks OK, but this part of the commit log is a
> bit misleading.
> Could you please modify the expression so that we can understand this
> patch fixes the issue when the disk image is corrupted and the leak
> wasn't always there ?

Makes sense. I'm going to fix the message as this:

When extending segment, the current segment is allocated and set dirty 
by previous nilfs_sufile_alloc().
But for some special cases such as corrupted image it can be unreliable, 
so nilfs_sufile_mark_dirty()
is called to promise that current segment is dirty.

However, nilfs_sufile_mark_dirty() only sets buffer and inode dirty 
while nilfs_segment_usage can
still be clean an used by following nilfs_sufile_alloc() because it 
checks nilfs_segment_usage_clean(su).

This will cause the problem reported...

Could you please have a check? Thanks!

Best,
Chen
> Originally, the assumption was that the current and next segments
> pointed to by log headers had been made dirty, and in fact mkfs.nilfs2
> and nilfs2 itself had created metadata that way, so it wasn't really a
> problem.  Usually the segment usage that this patch tries to dirty is
> already marked dirty and usually results in duplicate processing.
> nilfs_sufile_mark_dirty() is really only supposed to dirty that buffer
> and inode, and this patch changes the role.
>
> However, that assumption was incomplete in the sense that it does not
> assume broken metadata (whether intentionally or as a result of
> device/media failure), and lacked checks or protection from it.  In
> the meantime, you showed the simple and safe workaround even though it
> duplicates in almost all cases and even changes the semantics of the
> function.
> In terms of the stability and safety, your patch is good that we can
> ignore the inefficiency, so I am pushing for this change.
>
> Thanks,
> Ryusuke Konishi
>
> > 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.
> >
> > Fix this by setting usage as dirty in nilfs_sufile_mark_dirty(),
> > and add lock in it to protect the usage modification.
> >
> > Fixes: 9ff05123e3bf ("nilfs2: segment constructor")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: syzbot+77e4f005cb899d4268d1@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> > Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
> > Acked-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
> > Tested-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
> > ---
> > v1 -> v2:
> > 1) Add lock protection as Ryusuke suggested and slightly fix commit
> > message.
> > 2) Fix and add tags.
> > ---
> >   fs/nilfs2/sufile.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
> > index 77ff8e95421f..dc359b56fdfa 100644
> > --- a/fs/nilfs2/sufile.c
> > +++ b/fs/nilfs2/sufile.c
> > @@ -495,14 +495,22 @@ 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;
> >
> > +       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;
> >   }
> >
> > --
> > 2.17.1