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Re: use-after-free on log replay failure

To: Alex Lyakas <alex@xxxxxxxxxxxxxxxxx>
Subject: Re: use-after-free on log replay failure
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 12 Aug 2014 15:31:04 -0400
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <DC407F6E8F8C4EE1AF7117D7D6ABF282@alyakaslap>
References: <4B2A412C75324EE9880358513C069476@alyakaslap> <9D3CBECB663B4A77B7EF74B67973310A@alyakaslap> <20140804230721.GA20518@dastard> <AC10852F403846A182491ED8071135ED@alyakaslap> <20140806152042.GB39990@xxxxxxxxxxxxxxx> <CAOcd+r3bC59m7Rh-3tmjrnWnF5XoPQfE=U+=hz78NcAGu+Ou1g@xxxxxxxxxxxxxx> <20140811132057.GA1186@xxxxxxxxxxxxxxx> <20140811215207.GS20518@dastard> <20140812120341.GA46654@xxxxxxxxxxxxxxx> <DC407F6E8F8C4EE1AF7117D7D6ABF282@alyakaslap>
User-agent: Mutt/1.5.23 (2014-03-12)
On Tue, Aug 12, 2014 at 03:39:02PM +0300, Alex Lyakas wrote:
> Hello Dave, Brian,
> I will describe a generic reproduction that you ask for.
> 
> It was performed on pristine XFS code from 3.8.13, taken from here:
> git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
> 

This seems generic enough to me. Could you try on a more recent kernel?
Dave had mentioned there were fixes in this area of log recovery, so a
bisect might be all that is necessary to track down the patch you need.
Otherwise, we can pick up debugging from something more recent.

Brian

> top commit being:
> commit dbf932a9b316d5b29b3e220e5a30e7a165ad2992
> Author: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Date:   Sat May 11 13:57:46 2013 -0700
> 
>    Linux 3.8.13
> 
> 
> I made a single (I swear!) code change in XFS:
> 
> diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
> index 96fcbb8..d756bf6 100644
> --- a/fs/xfs/xfs_log_recover.c
> +++ b/fs/xfs/xfs_log_recover.c
> @@ -3736,6 +3736,9 @@ xlog_recover(
>        if ((error = xlog_find_tail(log, &head_blk, &tail_blk)))
>                return error;
> 
> +       xfs_notice(log->l_mp, "Sleep 10s before xlog_do_recover");
> +       msleep(10000);
> +
>        if (tail_blk != head_blk) {
>                /* There used to be a comment here:
>                 *
> 
> Fresh XFS was formatted on a 20 GB block device within a VM, using:
> mkfs.xfs -f -K /dev/vde -p /etc/zadara/xfs.protofile
> and:
> root@vc-00-00-1383-dev:~# cat /etc/zadara/xfs.protofile
> dummy                   : bootfilename, not used, backward compatibility
> 0 0                             : numbers of blocks and inodes, not used,
> backward compatibility
> d--777 0 0              : set 777 perms for the root dir
> $
> $
> 
> I mounted XFS with the following options:
> rw,sync,noatime,wsync,attr2,inode64,noquota 0 0
> 
> I started a couple of processes writing files sequentially onto this mount
> point, and after few seconds crashed the VM.
> When the VM came up, I took the metadump file and placed it in:
> https://drive.google.com/file/d/0ByBy89zr3kJNa0ZpdmZFS242RVU/edit?usp=sharing
> 
> Then I set up the following Device Mapper target onto /dev/vde:
> dmsetup create VDE --table "0 41943040 linear-custom /dev/vde 0"
> I am attaching the code (and Makefile) of dm-linear-custom target. It is
> exact copy of dm-linear, except that it has a module parameter. With the
> parameter set to 0, this is an identity mapping onto /dev/vde. If the
> parameter is set to non-0, all WRITE bios are failed with ENOSPC. There is a
> workqueue to fail them in a different context (not sure if really needed,
> but that's what our "real" custom
> block device does).
> 
> Now I did:
> mount -o noatime,sync /dev/mapper/VDE /mnt/xfs
> 
> The log recovery flow went into the sleep that I added, and then I did:
> echo 1 > /sys/module/dm_linear_custom/parameters/fail_writes
> 
> Problem reproduced:
> Aug 12 14:23:04 vc-00-00-1383-dev kernel: [  175.000657] XFS (dm-0):
> Mounting Filesystem
> Aug 12 14:23:04 vc-00-00-1383-dev kernel: [  175.026991] XFS (dm-0): Sleep
> 10s before xlog_do_recover
> Aug 12 14:23:14 vc-00-00-1383-dev kernel: [  185.028113] XFS (dm-0):
> Starting recovery (logdev: internal)
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556622] XFS (dm-0):
> metadata I/O error: block 0x2 ("xlog_recover_iodone") error 28 numblks 1
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556675] XFS (dm-0):
> metadata I/O error: block 0x40 ("xlog_recover_iodone") error 28 numblks 16
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556680] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556683] XFS (dm-0): I/O
> Error Detected. Shutting down filesystem
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556684] XFS (dm-0): Please
> umount the filesystem and rectify the problem(s)
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556766] XFS (dm-0):
> metadata I/O error: block 0xa00002 ("xlog_recover_iodone") error 5 numblks 1
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556769] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556771] XFS (dm-0):
> metadata I/O error: block 0xa00008 ("xlog_recover_iodone") error 5 numblks 8
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556774] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556776] XFS (dm-0):
> metadata I/O error: block 0xa00010 ("xlog_recover_iodone") error 5 numblks 8
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556779] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556781] XFS (dm-0):
> metadata I/O error: block 0xa00018 ("xlog_recover_iodone") error 5 numblks 8
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556783] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556785] XFS (dm-0):
> metadata I/O error: block 0xa00040 ("xlog_recover_iodone") error 5 numblks
> 16
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556788] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556790] XFS (dm-0):
> metadata I/O error: block 0xa00050 ("xlog_recover_iodone") error 5 numblks
> 16
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556793] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556880] XFS (dm-0):
> metadata I/O error: block 0xa00001 ("xlog_recover_iodone") error 28 numblks
> 1
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556884] XFS (dm-0):
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.556977] XFS (dm-0): log
> mount/recovery failed: error 28
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.557215] XFS (dm-0): log
> mount failed
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.573194] XFS ():
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.573214] XFS (): metadata
> I/O error: block 0x18 ("xlog_recover_iodone") error 28 numblks 8
> Aug 12 14:23:18 vc-00-00-1383-dev kernel: [  188.574685] XFS ():
> xfs_do_force_shutdown(0x1) called from line 377 of file
> /mnt/work/alex/linux-stable/source/fs/xfs/xfs_log_recover.c.  Return address
> = 0xffffffffa0349f68
> 
> As you see, after mount completes, IO callbacks are still arriving and
> printing junk (empty string in this case). Immediately after that kernel
> dies.
> 
> Is this description generic enough?
> 
> Thanks,
> Alex.
> 
> 
> 
> 
> -----Original Message----- From: Brian Foster
> Sent: 12 August, 2014 3:03 PM
> To: Dave Chinner
> Cc: Alex Lyakas ; xfs@xxxxxxxxxxx
> Subject: Re: use-after-free on log replay failure
> 
> On Tue, Aug 12, 2014 at 07:52:07AM +1000, Dave Chinner wrote:
> >On Mon, Aug 11, 2014 at 09:20:57AM -0400, Brian Foster wrote:
> >> On Sun, Aug 10, 2014 at 03:20:50PM +0300, Alex Lyakas wrote:
> >> > On Wed, Aug 6, 2014 at 6:20 PM, Brian Foster <bfoster@xxxxxxxxxx> > >
> >wrote:
> >> > > On Wed, Aug 06, 2014 at 03:52:03PM +0300, Alex Lyakas wrote:
> >.....
> >> > >> But I believe, my analysis shows that during the mount sequence XFS
> >> > >> does not
> >> > >> wait properly for all the bios to complete, before failing the > >
> >>> mount
> >> > >> sequence back to the caller.
> >> > >>
> >> > >
> >> > > As an experiment, what about the following? Compile tested only and
> >> > > not
> >> > > safe for general use.
> >...
> >> > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> >> > > index cd7b8ca..fbcf524 100644
> >> > > --- a/fs/xfs/xfs_buf.c
> >> > > +++ b/fs/xfs/xfs_buf.c
> >> > > @@ -1409,19 +1409,27 @@ xfs_buf_iorequest(
> >> > >   * case nothing will ever complete.  It returns the I/O error code,
> >> > > if any, or
> >> > >   * 0 if there was no error.
> >> > >   */
> >> > > -int
> >> > > -xfs_buf_iowait(
> >> > > -       xfs_buf_t               *bp)
> >> > > +static int
> >> > > +__xfs_buf_iowait(
> >> > > +       struct xfs_buf          *bp,
> >> > > +       bool                    skip_error)
> >> > >  {
> >> > >         trace_xfs_buf_iowait(bp, _RET_IP_);
> >> > >
> >> > > -       if (!bp->b_error)
> >> > > +       if (skip_error || !bp->b_error)
> >> > >                 wait_for_completion(&bp->b_iowait);
> >> > >
> >> > >         trace_xfs_buf_iowait_done(bp, _RET_IP_);
> >> > >         return bp->b_error;
> >> > >  }
> >> > >
> >> > > +int
> >> > > +xfs_buf_iowait(
> >> > > +       struct xfs_buf          *bp)
> >> > > +{
> >> > > +       return __xfs_buf_iowait(bp, false);
> >> > > +}
> >> > > +
> >> > >  xfs_caddr_t
> >> > >  xfs_buf_offset(
> >> > >         xfs_buf_t               *bp,
> >> > > @@ -1866,7 +1874,7 @@ xfs_buf_delwri_submit(
> >> > >                 bp = list_first_entry(&io_list, struct xfs_buf, > >
> >> b_list);
> >> > >
> >> > >                 list_del_init(&bp->b_list);
> >> > > -               error2 = xfs_buf_iowait(bp);
> >> > > +               error2 = __xfs_buf_iowait(bp, true);
> >> > >                 xfs_buf_relse(bp);
> >> > >                 if (!error)
> >> > >                         error = error2;
> >
> >Not waiting here on buffer error should not matter. Any buffer that
> >is under IO and requires completion should be referenced, and that
> >means it should be caught and waited on by xfs_wait_buftarg() in the
> >mount failure path after log recovery fails.
> >
> 
> I think that assumes the I/O is successful. Looking through
> xlog_recover_buffer_pass2() as an example, we read the buffer which
> should return with b_hold == 1. The delwri queue increments the hold and
> we xfs_buf_relse() in the return path (i.e., buffer is now held by the
> delwri queue awaiting submission).
> 
> Sometime later we delwri_submit()... xfs_buf_iorequest() does an
> xfs_buf_hold() and xfs_buf_rele() within that single function. The
> delwri_submit() releases its hold after xfs_buf_iowait(), which I guess
> at that point bp should go onto the lru (b_hold back to 1 in
> xfs_buf_rele(). Indeed, the caller has lost scope of the buffer at this
> point.
> 
> So unless I miss something or got the lifecycle wrong here, which is
> easily possible ;), this all hinges on xfs_buf_iowait(). That's where
> the last hold forcing the buffer to stay around goes away.
> xfs_buftarg_wait_rele() will dispose the buffer if b_hold == 1. If
> xfs_buf_iowait() is racy in the event of I/O errors via the bio
> callback, I think this path is susceptible just the same.
> 
> >> > > ---
> >> > I think that this patch fixes the problem. I tried reproducing it like
> >> > 30 times, and it doesn't happen with this patch. Dropping this patch
> >> > reproduces the problem within 1 or 2 tries. Thanks!
> >> > What are next steps? How to make it "safe for general use"?
> >> >
> >>
> >> Ok, thanks for testing. I think that implicates the caller bypassing the
> >> expected blocking with the right sequence of log recovery I/Os and
> >> device failure. TBH, I'd still like to see the specifics, if possible.
> >> Could you come up with a generic reproducer for this? I think a metadump
> >> of the fs with the dirty log plus whatever device failure simulation
> >> hack you're using would suffice.
> >
> >The real issue is we don't know exactly what code is being tested
> >(it's 3.8 + random bug fix backports + custom code). Even if we have
> >a reproducer there's no guarantee it will reproduce on a current
> >kernel. IOWs, we are stumbling around in the dark bashing our heads
> >against everything in the room, and that just wastes everyone's
> >time.
> >
> >We need a reproducer that triggers on a current, unmodified
> >kernel release. You can use dm-faulty to error out all writes just
> >like you are doing with your custom code. See
> >xfstests::tests/generic/321 and common/dmflakey for to do this.
> >Ideally the reproducer is in a form that xfstests can use....
> >
> >If you can't reproduce it on an upstream kernel, then git bisect is
> >your friend. It will find the commit that fixed the problem you are
> >seeing....
> >
> 
> Ugh, yeah. The fact that this was customized as such apparently went
> over my head. I agree completely. This needs to be genericized to a
> pristine, preferably current kernel. The experiment patch could be
> papering over something completely different.
> 
> >> The ideal fix is not yet clear to me.
> >
> >We are not even that far along - the root cause of the bug is not at
> >all clear to me. :/
> >
> 
> Yeah.. the above was just the theory that motivated the experiment in
> the previously posted patch. It of course remains a theory until we can
> see the race in action. I was referring to the potential fix for the
> raciness of xfs_buf_iowait() with regard to bio errors and the wq iodone
> handling, while still asking for a reproducer to confirm the actual
> problem. FWIW, I'm not too high on changes in the buf management code,
> even a smallish behavior change, without a real trace of some sort that
> documents the problem and justifies the change.
> 
> Brian
> 
> >Cheers,
> >
> >Dave.
> >-- 
> >Dave Chinner
> >david@xxxxxxxxxxxxx



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