Re: [PATCH v2] iomap: zero cached page over unwritten extent on truncate page

[Brian Foster <bfoster@redhat.com>]

On Wed, Oct 21, 2020 at 09:25:47AM -0700, Darrick J. Wong wrote:
> On Wed, Oct 21, 2020 at 09:33:29AM -0400, Brian Foster wrote:
> > iomap_truncate_page() relies on zero range and zero range
> > unconditionally skips unwritten mappings. This is normally not a
> > problem as most users synchronize in-core state to the underlying
> > block mapping by flushing pagecache prior to calling into iomap.
> > This is not the case for iomap_truncate_page(), however. XFS calls
> > iomap_truncate_page() on truncate down before flushing the new EOF
> > page of the file. This means that if the new EOF block is unwritten
> > but covered by a dirty or writeback page (i.e. awaiting unwritten
> > conversion after writeback), iomap fails to zero that page. The
> > subsequent truncate_setsize() call does perform page zeroing, but
> > doesn't dirty the page. Therefore if the new EOF page is written
> > back after calling into iomap but before the pagecache truncate, the
> > post-EOF zeroing is lost on page reclaim. This exposes stale
> > post-EOF data on mapped reads.
> > 
> > Rework iomap_truncate_page() to check pagecache state before calling
> > into iomap_apply() and use that info to determine whether we can
> > safely skip zeroing unwritten extents. The filesystem has locked out
> > concurrent I/O and mapped operations at this point but is not
> > serialized against writeback, unwritten extent conversion (I/O
> > completion) or page reclaim. Therefore if a page does not exist
> > before we acquire the mapping, we can be certain that an unwritten
> > extent cannot be converted before we return and thus it is safe to
> > skip. If a page does exist over an unwritten block, it could be in
> > the dirty or writeback states, convert the underlying mapping at any
> > time, and thus should be explicitly written to avoid racing with
> > writeback. Finally, since iomap_truncate_page() only targets the new
> > EOF block and must now pass additional state to the actor, open code
> > the zeroing actor instead of plumbing through zero range.
> > 
> > This does have the tradeoff that an existing clean page is dirtied
> > and causes unwritten conversion, but this is analogous to historical
> > behavior implemented by block_truncate_page(). This patch restores
> > historical behavior to address the data exposure problem and leaves
> > filtering out the clean page case for a separate patch.
> > Fixes: 68a9f5e7007c ("xfs: implement iomap based buffered write path")
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > v2:
> > - Rework to check for cached page explicitly and avoid use of seek data.
> > v1: https://lore.kernel.org/linux-fsdevel/20201012140350.950064-1-bfoster@redhat.com/
> 
> Has the reproducer listed in that email been turned into a fstest case
> yet? :)
> 

Heh.. that reproducer actually required customization to manufacture the
problem. I'll have to think more about a generic reproducer.

FWIW, I've also just come across a similar post-eof data exposure
failure with this patch, so I'll need to dig into that first and
foremost and figure out whether this is still incorrect/insufficient for
some reason...

> > 
> >  fs/iomap/buffered-io.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 40 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index bcfc288dba3f..2cdfcff02307 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -1000,17 +1000,56 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> >  }
> >  EXPORT_SYMBOL_GPL(iomap_zero_range);
> >  
> > +struct iomap_trunc_priv {
> > +	bool *did_zero;
> > +	bool has_page;
> > +};
> > +
> > +static loff_t
> > +iomap_truncate_page_actor(struct inode *inode, loff_t pos, loff_t count,
> > +		void *data, struct iomap *iomap, struct iomap *srcmap)
> > +{
> > +	struct iomap_trunc_priv	*priv = data;
> > +	unsigned offset;
> > +	int status;
> > +
> > +	if (srcmap->type == IOMAP_HOLE)
> > +		return count;
> > +	if (srcmap->type == IOMAP_UNWRITTEN && !priv->has_page)
> > +		return count;
> > +
> > +	offset = offset_in_page(pos);
> > +	if (IS_DAX(inode))
> > +		status = dax_iomap_zero(pos, offset, count, iomap);
> > +	else
> > +		status = iomap_zero(inode, pos, offset, count, iomap, srcmap);
> > +	if (status < 0)
> > +		return status;
> > +
> > +	if (priv->did_zero)
> > +		*priv->did_zero = true;
> > +	return count;
> > +}
> > +
> >  int
> >  iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
> >  		const struct iomap_ops *ops)
> >  {
> > +	struct iomap_trunc_priv priv = { .did_zero = did_zero };
> >  	unsigned int blocksize = i_blocksize(inode);
> >  	unsigned int off = pos & (blocksize - 1);
> > +	loff_t ret;
> >  
> >  	/* Block boundary? Nothing to do */
> >  	if (!off)
> >  		return 0;
> > -	return iomap_zero_range(inode, pos, blocksize - off, did_zero, ops);
> > +
> > +	priv.has_page = filemap_range_has_page(inode->i_mapping, pos, pos);
> 
> Er... shouldn't that second 'pos' be 'pos + blocksize - off - 1', like
> the apply call below?  I guess it doesn't matter since we're only
> interested in the page at pos, but the double usage of pos caught my
> eye.
> 

Yeah. I'll fix that up.

> I also wonder, can you move this into the actor so that you can pass
> *did_zero straight through without the two-item struct?
> 

I don't think so because the idea was to explicitly check for page
presence prior to getting the mapping.

Brian

> --D
> 
> > +	ret = iomap_apply(inode, pos, blocksize - off, IOMAP_ZERO, ops, &priv,
> > +			  iomap_truncate_page_actor);
> > +	if (ret <= 0)
> > +		return ret;
> > +	return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(iomap_truncate_page);
> >  
> > -- 
> > 2.25.4
> > 
>