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Re: [ 68/89] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [ 68/89] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem end
From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 14 Feb 2013 11:26:14 -0800
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, Dave Chinner <dchinner@xxxxxxxxxx>, Brian Foster <bfoster@xxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, CAI Qian <caiqian@xxxxxxxxxx>, xfs@xxxxxxxxxxx
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On Thu, Feb 14, 2013 at 11:07:30AM +1100, Dave Chinner wrote:
> [cc xfs@xxxxxxxxxxx]
> On Wed, Feb 13, 2013 at 08:18:45AM -0800, Greg Kroah-Hartman wrote:
> > On Wed, Feb 13, 2013 at 04:30:32PM +0100, Paolo Bonzini wrote:
> > > Il 01/02/2013 14:08, Greg Kroah-Hartman ha scritto:
> > > > 3.7-stable review patch.  If anyone has any objections, please let me 
> > > > know.
> > > > 
> > > > ------------------
> > > > 
> > > > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > > > 
> > > > commit eb178619f930fa2ba2348de332a1ff1c66a31424 upstream.
> > > > 
> > > > When _xfs_buf_find is passed an out of range address, it will fail
> > > > to find a relevant struct xfs_perag and oops with a null
> > > > dereference. This can happen when trying to walk a filesystem with a
> > > > metadata inode that has a partially corrupted extent map (i.e. the
> > > > block number returned is corrupt, but is otherwise intact) and we
> > > > try to read from the corrupted block address.
> > > > 
> > > > In this case, just fail the lookup. If it is readahead being issued,
> > > > it will simply not be done, but if it is real read that fails we
> > > > will get an error being reported.  Ideally this case should result
> > > > in an EFSCORRUPTED error being reported, but we cannot return an
> > > > error through xfs_buf_read() or xfs_buf_get() so this lookup failure
> > > > may result in ENOMEM or EIO errors being reported instead.
> > > 
> > > It looks like this breaks xfs_growfs.  See
> > > http://bugzilla.redhat.com/show_bug.cgi?id=909602.
> Entirely possible, as the filesystem size is not updated until after
> all the new metadata is written to disk. in 3.8, there's this commit:
> commit fd23683c3b1ab905cba61ea2981c156f4bf52845
> Author: Dave Chinner <dchinner@xxxxxxxxxx>
> Date:   Mon Nov 12 22:53:59 2012 +1100
>     xfs: growfs: use uncached buffers for new headers
>     When writing the new AG headers to disk, we can't attach write
>     verifiers because they have a dependency on the struct xfs-perag
>     being attached to the buffer to be fully initialised and growfs
>     can't fully initialise them until later in the process.
>     The simplest way to avoid this problem is to use uncached buffers
>     for writing the new headers. These buffers don't have the xfs-perag
>     attached to them, so it's simple to detect in the write verifier and
>     be able to skip the checks that need the xfs-perag.
>     This enables us to attach the appropriate buffer ops to the buffer
>     and henc calculate CRCs on the way to disk. IT also means that the
>     buffer is torn down immediately, and so the first access to the AG
>     headers will re-read the header from disk and perform full
>     verification of the buffer. This way we also can catch corruptions
>     due to problems that went undetected in growfs.
>     Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
>     Reviewed-by Rich Johnston <rjohnston@xxxxxxx>
>     Signed-off-by: Ben Myers <bpm@xxxxxxx>
> As part of the metadata verifier feature. It means that growfs no
> longer uses cached buffers, and hence does not pass through
> _xfs_buf_find() and hence will not trigger the beyond-EOFS that the
> above commit adds.
> > Ick, not good.
> > 
> > Dave, any thoughts here?  Should I drop this from the 3.7-stable queue?
> Yeah, drop it.
> But what I'm now wondering is how this patch got proposed for
> 3.7-stable. I don't recall seeing anything about this being
> proposed.
> <trolls email archives>
> Oh, it happened while I was at LCA and didn't have any access to Red
> Hat email and there was a private thread about it. By the time I
> read it the stable kernel was already released and so it immediately
> dropped from my attention.
> XFS Maintainers: Major process fail. Patches that are being proposed
> for backports need to be posted to the XFS list, reviewed and tested
> before saying they are OK to go.  We have several growfs tests in
> xfstests would have failed if this was actually tested.
> Stable folk: This is the reason why I, quite frankly, don't want to
> support stable kernels *at all*. The overhead of backporting and
> testing a patch to a single kernel target to ensure there are no
> unintended regressions is significant, and there are so many stable
> kernels no it's just a waste of developer time to try to support
> them. And in this case, the process simply wasn't executed and an
> unintended regression that is >this close< to causing filesystem
> corruption slipped through to the stable series.....

Ok, how about I never apply any xfs stable kernel patch, unless you send
it to stable@xxxxxxxxxxxxxxx?  I have that rule in place for some other
subsystems that don't want me applying stuff that they aren't aware of,
and have no problem doing the same thing here.

Just let me know.

I'll go revert this patch for the next 3.7-stable release.


greg k-h

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