Re: [PATCH 3/3] xfs: Fix stale data exposure when readahead races with hole punch

[Jan Kara <jack@suse.cz>]

On Fri 30-08-19 17:24:49, Jan Kara wrote:
> On Thu 29-08-19 08:52:04, Darrick J. Wong wrote:
> > On Thu, Aug 29, 2019 at 03:10:34PM +0200, Jan Kara wrote:
> > > Hole puching currently evicts pages from page cache and then goes on to
> > > remove blocks from the inode. This happens under both XFS_IOLOCK_EXCL
> > > and XFS_MMAPLOCK_EXCL which provides appropriate serialization with
> > > racing reads or page faults. However there is currently nothing that
> > > prevents readahead triggered by fadvise() or madvise() from racing with
> > > the hole punch and instantiating page cache page after hole punching has
> > > evicted page cache in xfs_flush_unmap_range() but before it has removed
> > > blocks from the inode. This page cache page will be mapping soon to be
> > > freed block and that can lead to returning stale data to userspace or
> > > even filesystem corruption.
> > > 
> > > Fix the problem by protecting handling of readahead requests by
> > > XFS_IOLOCK_SHARED similarly as we protect reads.
> > > 
> > > CC: stable@vger.kernel.org
> > > Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahcyeaEVOFKVQ5dw@mail.gmail.com/
> > > Reported-by: Amir Goldstein <amir73il@gmail.com>
> > > Signed-off-by: Jan Kara <jack@suse.cz>
> > 
> > Is there a test on xfstests to demonstrate this race?
> 
> No, but I can try to create one.

I was experimenting with this but I could not reproduce the issue in my
test VM without inserting artificial delay at appropriate place... So I
don't think there's much point in the fstest for this.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR