Re: [ 68/89] xfs: fix _xfs_buf_find oops on blocks beyond thefilesystem end

[Ben Myers]

Greg,

On Thu, Feb 14, 2013 at 11:26:14AM -0800, Greg Kroah-Hartman wrote:
> 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?

Dave has made it clear that he doesn't want to be involved in maintaining
-stable kernels.  However, my team at SGI is interested in maintaining -stable
kernels.  We're not going to use the fact that there is a risk of regression as
an excuse to starve -stable of relevant fixes, just as we do not use it as an
excuse to starve the upstream branch of feature content.

> 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.

Here are the usual suspects:

Ben Myers <bpm@xxxxxxx>
Mark Tinguely <tinguely@xxxxxxx>
Dave Chinner <dchinner@xxxxxxxxxx>
Eric Sandeen <sandeen@xxxxxxxxxx>

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

Much appreciated.

Regards,
	Ben
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