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

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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@xxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahcyeaEVOFKVQ5dw@xxxxxxxxxxxxxx/
> > Reported-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> 
> Is there a test on xfstests to demonstrate this race?

No, but I can try to create one.

> Will test it out though...
> 
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

Thanks. BTW, will you pick up these patches please?

								Honza

> 
> --D
> 
> > ---
> >  fs/xfs/xfs_file.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 28101bbc0b78..d952d5962e93 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -28,6 +28,7 @@
> >  #include <linux/falloc.h>
> >  #include <linux/backing-dev.h>
> >  #include <linux/mman.h>
> > +#include <linux/fadvise.h>
> >  
> >  static const struct vm_operations_struct xfs_file_vm_ops;
> >  
> > @@ -933,6 +934,30 @@ xfs_file_fallocate(
> >  	return error;
> >  }
> >  
> > +STATIC int
> > +xfs_file_fadvise(
> > +	struct file	*file,
> > +	loff_t		start,
> > +	loff_t		end,
> > +	int		advice)
> > +{
> > +	struct xfs_inode *ip = XFS_I(file_inode(file));
> > +	int ret;
> > +	int lockflags = 0;
> > +
> > +	/*
> > +	 * Operations creating pages in page cache need protection from hole
> > +	 * punching and similar ops
> > +	 */
> > +	if (advice == POSIX_FADV_WILLNEED) {
> > +		lockflags = XFS_IOLOCK_SHARED;
> > +		xfs_ilock(ip, lockflags);
> > +	}
> > +	ret = generic_fadvise(file, start, end, advice);
> > +	if (lockflags)
> > +		xfs_iunlock(ip, lockflags);
> > +	return ret;
> > +}
> >  
> >  STATIC loff_t
> >  xfs_file_remap_range(
> > @@ -1232,6 +1257,7 @@ const struct file_operations xfs_file_operations = {
> >  	.fsync		= xfs_file_fsync,
> >  	.get_unmapped_area = thp_get_unmapped_area,
> >  	.fallocate	= xfs_file_fallocate,
> > +	.fadvise	= xfs_file_fadvise,
> >  	.remap_file_range = xfs_file_remap_range,
> >  };
> >  
> > -- 
> > 2.16.4
> > 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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