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Date: Sat, 2 Apr 2016 11:46:34 -0400
From: Theodore Ts'o <tytso@....edu>
To: Yves-Alexis Perez <corsac@...ian.org>
Cc: oss-security@...ts.openwall.com, Johannes Segitz <jsegitz@...e.com>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: ext4 data corruption due to punch hole races

On Sat, Apr 02, 2016 at 03:14:57PM +0200, Yves-Alexis Perez wrote:
> > "When punching holes into a file races with the page fault of the same
> > area, it is possible that freed blocks remain referenced from page cache
> > pages mapped to process' address space. Thus modification of these blocks
> > can corrupt data someone else is now storing in those blocks (which
> > obviously has security implications if you can trick filesystem into
> > storing some important file in those blocks).
> > 
> > This affects all the kernels where we support ext4 for writing. Relevant
> > fixes upstream are commits ea3d7209ca01da209cda6f0dea8be9cc4b7a933b,
> > 17048e8a083fec7ad841d88ef0812707fbc7e39f,
> > 32ebffd3bbb4162da5ff88f9a35dd32d0a28ea70,
> > 011278485ecc3cd2a3954b5d4c73101d919bf1fa."
> 
> any reason why those commits weren't CC: stable? If this really affects all
> kernels where ext4 writing is possible, that means basically all current
> stable kernels more or less, I guess?

They weren't cc'ed stable because they're fairly complex patches,
which (a) means they probably wouldn't auto-apply anyway, and (b)
someone who does do the (probably manual) back port they would be
*very* strongly advised to run them through a complete ext4 regression
test series[1] to make sure the patches actually don't make things
worse from a stability perspective.

[1] http://thunk.org/gce-xfstests

I do spend *small* amount of work testing the stable kernels (3.10,
3.14, 3.18, 4.1, 4.4) using gce-xfstests and backporting and testing
patches that weren't cc'ed to stable for various reasons.  It's a
pretty low priority task, though, and I'd really love to delegate this
to someone else.  I just don't have the bandwidth to support back
level kernels (this is why distributions get paid the big bucks), and
note that even if I or someone else stepped up, this won't necessarily
help Debian, which isn't on a one of the stable kernel versions.

If anyone is interested, please contact me.  Otherwise, I'll get to it
eventually.

						- Ted

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