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In-Reply-To: <5c8dd545-2190-162e-a9de-2323fcad716f@huawei.com>
From:   Ryusuke Konishi <konishi.ryusuke@gmail.com>
Date:   Tue, 8 Nov 2022 19:33:49 +0900
Message-ID: <CAKFNMokcSj9YSLeXm=S4rY5nMx6DjQvRHXVaLVu2CbNEia7-2Q@mail.gmail.com>
Subject: Re: [PATCH] nilfs2: fix NULL pointer dereference in nilfs_segctor_prepare_write()
To:     Liu Shixin <liushixin2@huawei.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org
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On Tue, Nov 8, 2022 at 3:49 PM Liu Shixin wrote:
>
>
>
> On 2022/11/8 12:41, Ryusuke Konishi wrote:
> > Hi Liu Shixin,
> >
> > On Tue, Nov 8, 2022 at 10:41 AM Liu Shixin wrote:
> >> Syzbot reported a NULL pointer dereference:
> >>
> >>  Unable to handle kernel NULL pointer dereference at virtual address 0=
000000000000168
> >>  Mem abort info:
> >>    ESR =3D 0x0000000096000004
> >>    EC =3D 0x25: DABT (current EL), IL =3D 32 bits
> >>    SET =3D 0, FnV =3D 0
> >>    EA =3D 0, S1PTW =3D 0
> >>    FSC =3D 0x04: level 0 translation fault
> >>  Data abort info:
> >>    ISV =3D 0, ISS =3D 0x00000004
> >>    CM =3D 0, WnR =3D 0
> >>  user pgtable: 4k pages, 48-bit VAs, pgdp=3D0000000108bcf000
> >>  [0000000000000168] pgd=3D0000000000000000, p4d=3D0000000000000000
> >>  Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
> >>  Modules linked in:
> >>  CPU: 1 PID: 3032 Comm: segctord Not tainted 6.0.0-rc7-syzkaller-18095=
-gbbed346d5a96 #0
> >>  Hardware name: Google Google Compute Engine/Google Compute Engine, BI=
OS Google 09/30/2022
> >>  pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=3D--)
> >>  pc : _compound_head include/linux/page-flags.h:253 [inline]
> >>  pc : lock_page+0x28/0x1e0 include/linux/pagemap.h:958
> >>  lr : lock_page+0x28/0x1e0 include/linux/pagemap.h:956
> >>  sp : ffff80001290bc00
> >>  x29: ffff80001290bc00 x28: ffff80001290bde0 x27: 000000000000001b
> >>  x26: fffffc000330d7c0 x25: ffff0000caa56d68 x24: ffff0000ca9fb1c0
> >>  x23: 0000000000000080 x22: ffff0000ca9fb130 x21: 0000000000000160
> >>  x20: ffff0000c91e10b8 x19: 0000000000000160 x18: 00000000000000c0
> >>  x17: ffff80000dd0b198 x16: ffff80000db49158 x15: ffff0000c3e63500
> >>  x14: 0000000000000000 x13: 00000000ffffffff x12: ffff0000c3e63500
> >>  x11: ff808000095d1a0c x10: 0000000000000000 x9 : 0000000000000000
> >>  x8 : 0000000000000000 x7 : ffff80000856806c x6 : 0000000000000000
> >>  x5 : 0000000000000080 x4 : 0000000000000000 x3 : 0000000000000000
> >>  x2 : 0000000000000000 x1 : ffff80000cb431b1 x0 : 0000000000000000
> >>  Call trace:
> >>   lock_page+0x28/0x1e0 include/linux/pagemap.h:956
> >>   nilfs_segctor_prepare_write+0x6c/0x21c fs/nilfs2/segment.c:1658
> >>   nilfs_segctor_do_construct+0x9f4/0xee8 fs/nilfs2/segment.c:2068
> >>   nilfs_segctor_construct+0xa0/0x380 fs/nilfs2/segment.c:2375
> >>   nilfs_segctor_thread_construct fs/nilfs2/segment.c:2483 [inline]
> >>   nilfs_segctor_thread+0x180/0x660 fs/nilfs2/segment.c:2566
> >>   kthread+0x12c/0x158 kernel/kthread.c:376
> >>   ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:860
> >>
> >> If didn't call nilfs_sufile_alloc() in nilfs_segctor_begin_constructio=
n(),
> >> nilfs_sufile_header's sh_last_alloc is not updated. In such case, we w=
ill
> >> add a bh in two segbuf->sb_segsum_buffers. And finally cause list erro=
r.
> >>
> >> Reported-by: syzbot+77e4f005cb899d4268d1@syzkaller.appspotmail.com
> >> Fixes: 9ff05123e3bf ("nilfs2: segment constructor")
> >> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> >> ---
> >>  fs/nilfs2/segment.c | 1 +
> >>  fs/nilfs2/sufile.c  | 2 +-
> >>  2 files changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
> >> index b4cebad21b48..7be632c15f91 100644
> >> --- a/fs/nilfs2/segment.c
> >> +++ b/fs/nilfs2/segment.c
> >> @@ -1371,6 +1371,7 @@ static int nilfs_segctor_extend_segments(struct =
nilfs_sc_info *sci,
> >>                 sci->sc_segbuf_nblocks +=3D segbuf->sb_rest_blocks;
> >>
> >>                 /* allocate the next next full segment */
> >> +               nextnextnum =3D segbuf->sb_segnum;
> >>                 err =3D nilfs_sufile_alloc(sufile, &nextnextnum);
> >>                 if (unlikely(err))
> >>                         goto failed_segbuf;
> >> diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
> >> index 77ff8e95421f..853a8212114f 100644
> >> --- a/fs/nilfs2/sufile.c
> >> +++ b/fs/nilfs2/sufile.c
> >> @@ -317,7 +317,7 @@ int nilfs_sufile_alloc(struct inode *sufile, __u64=
 *segnump)
> >>                 goto out_sem;
> >>         kaddr =3D kmap_atomic(header_bh->b_page);
> >>         header =3D kaddr + bh_offset(header_bh);
> >> -       last_alloc =3D le64_to_cpu(header->sh_last_alloc);
> >> +       last_alloc =3D max(le64_to_cpu(header->sh_last_alloc), *segnum=
p);
> >>         kunmap_atomic(kaddr);
> >>
> >>         nsegments =3D nilfs_sufile_get_nsegments(sufile);
> >> --
> >> 2.25.1
> > Thank you for your help.   I have a few questions, so I'll ask them bel=
ow.
> >
> >> If didn't call nilfs_sufile_alloc() in nilfs_segctor_begin_constructio=
n(),
> >> nilfs_sufile_header's sh_last_alloc is not updated. In such case, we w=
ill
> >> add a bh in two segbuf->sb_segsum_buffers.
> > If nilfs_sufile_alloc() succeeds to allocate a segment, sh_last_alloc
> > is updated.
> > all segment allocation must be done through nilfs_sufile_alloc().
> > And, the allocated segment is marked dirty on the sufile not to be
> > reallocated until it's freed.
> >
> > So, why is it happening that the same segment is allocated twice in a l=
og ?
> > Is it hard to fix the problem by correcting the calling sequence of
> > nilfs_sufile_alloc()/free()/etc without touching nilfs_sufile_alloc()
> > ?
> The problem happened when we call nilfs_segctor_begin_construction() and =
satisfied
> condition nilfs->ns_segnum !=3D nilfs->ns_nextnum. In such scenario, nilf=
s_sufile_alloc()
> will be skipped, but we call nilfs_segbuf_map() and nilfs_segbuf_set_next=
_segnum()
> all the time, so last_alloc is not updated.
> Then in nilfs_segctor_extend_segments(), we set sb_segnum by prev->sb_nex=
tnum directly,
> and calculate next sb_segnum by nilfs_sufile_alloc(), since last_alloc is=
 not updated,
> we will get sb_segnum again.
>
> By the way, I still don't understand why skip nilfs_sufile_alloc() in som=
e cases and why
> nilfs->ns_segnum !=3D nilfs->ns_nextnum. Do you have any ideas?
> >
> > I haven't looked closely at this patch yet, but I'm concerned about
> > the impact on other places as well.
> > nilfs_sufile_alloc() is also used in
> > nilfs_segctor_begin_construction() and
> > nilfs_prepare_segment_for_recovery().  Are there any side effects?
> >
> > This patch turns an output-only argument into both input and output,
> > and that input value is always used in the calculation of
> > "last_alloc".
> > So, this change requires all callers to pass a meaningful initial
> > value (at least a valid value) to *segnump.
> >
> > Another question, will this work near the end of the segments ?
> > Since segments are used cyclically, wouldn't comparison with the max
> > function break down there?
> > (I mean it seems that sh_last_alloc may be chosen unintentionally at th=
e end.)
> Thanks for the heads-up=EF=BC=8CI need to look at it again. This patch ca=
n only prevent this problem,
> and seems to need improvement. Maybe there is a more reasonable solution.
>
> Thanks,

I will explain step by step.

> By the way, I still don't understand why skip nilfs_sufile_alloc() in som=
e cases and why
> nilfs->ns_segnum !=3D nilfs->ns_nextnum. Do you have any ideas?

nilfs->ns_segnum holds the segment number where nilfs2 currently
writes logs to, and nilfs->ns_nextnum holds the next segment number
where nilfs2 will select to write logs when the current segment gets
full.
Here, a segment is a fixed sized container of logs (*), in which
multiple logs can be appended and written in.
(*) Exceptionally, segment #0 is shorter than others to exclude the
superblock area.

The reason "nextnum" is needed is that mount-time recovery processing
needs to know which segment the logs follow.

Both segments (nilfs->ns_segnum and nilfs->ns_nextnum) must be already
allocated on the sufile.
Here, "allocated" means the corresponding segment usage entry is
marked "dirty" on the sufile.
In nilfs_sufile_alloc() this is done with the following line:

                        nilfs_segment_usage_set_dirty(su);

On the other hand, the test whether the segment is allocatable or not
is done at the following part:

                        if (!nilfs_segment_usage_clean(su))
                                continue;
                        /* found a clean segment */

Coming back to your last question, if nilfs->ns_segnum !=3D
nilfs->ns_nextnum, it indicates that nilfs2 is still writing a log to
the segment of ns_segnum.
Both should be in the allocated state, so there is no need to call
nilfs_sufile_alloc() in this case.

If nilfs->ns_segnum =3D=3D nilfs->ns_nextnum in
nilfs_segctor_begin_construction(), then nilfs_shift_to_next_segment()
sets ns_segnum to ns_nextnum because the current segment (ns_segnum)
is full and no more logs can be written.
In this case, a new segment must be allocated for new ns_nextnum,
therefore nilfs_sufile_alloc() will be called.

> The problem happened when we call nilfs_segctor_begin_construction() and =
satisfied
> condition nilfs->ns_segnum !=3D nilfs->ns_nextnum. In such scenario, nilf=
s_sufile_alloc()
> will be skipped, but we call nilfs_segbuf_map() and nilfs_segbuf_set_next=
_segnum()
> all the time, so last_alloc is not updated.

As I explained above, it is expected behavior that
nilfs_sufile_alloc() is not called and last_alloc is not updated if
nilfs->ns_segnum !=3D nilfs->ns_nextnum.

nilfs_segbuf_map() is always called there to set the geometry
information in the segbuf.  This is expected behavior.
And, nilfs_segbuf_set_next_segnum() is called later to set the
determined ns_nextnum to the segbuf, so this is expected behavior as
well.

> Then in nilfs_segctor_extend_segments(), we set sb_segnum by prev->sb_nex=
tnum directly,
> and calculate next sb_segnum by nilfs_sufile_alloc(), since last_alloc is=
 not updated,
> we will get sb_segnum again.

nilfs_segctor_extend_segments() pre-allocates one or more segments
large enough to store updated blocks of metadata files that need to be
written in a series of logs at once, and sets up a chain of segbufs.
(Sorry for the missing function comment).

sb_segnum is set by prev->sb_nextnum to form a chain of buffers for
segments.  This is expected behavior.
And, the sb_nextnum (=3D next sb_segnum) will be given by
nilfs_sufile_alloc().   This is also expected.
It looks like the problem is that nilfs_sufile_alloc() here allocates
the same segnum again.

Because sb_segnum is set by prev->sb_nextnum which is allocated by the
previous nilfs_sufile_alloc() call,
this usually does not happen.

A possible anomaly is if the segment pointed by the first nextnum (or
segnum) was not marked dirty on sufile.
This may happen if the sufile is corrupted on the test disk image that
syzbot provided (mounted).

Can you confirm if this is actually happening?

Regards,
Ryusuke Konishi