Re: [PATCH] xfs: fix attr leaf header freemap.size underflow

[Brian Foster <bfoster@redhat.com>]

On Fri, Nov 15, 2019 at 10:59:55AM -0800, Darrick J. Wong wrote:
> On Fri, Nov 15, 2019 at 06:41:37AM -0500, Brian Foster wrote:
> > The leaf format xattr addition helper xfs_attr3_leaf_add_work()
> > adjusts the block freemap in a couple places. The first update drops
> > the size of the freemap that the caller had already selected to
> > place the xattr name/value data. Before the function returns, it
> > also checks whether the entries array has encroached on a freemap
> > range by virtue of the new entry addition. This is necessary because
> > the entries array grows from the start of the block (but end of the
> > block header) towards the end of the block while the name/value data
> > grows from the end of the block in the opposite direction. If the
> > associated freemap is already empty, however, size is zero and the
> > subtraction underflows the field and causes corruption.
> > 
> > This is reproduced rarely by generic/070. The observed behavior is
> > that a smaller sized freemap is aligned to the end of the entries
> > list, several subsequent xattr additions land in larger freemaps and
> > the entries list expands into the smaller freemap until it is fully
> > consumed and then underflows. Note that it is not otherwise a
> > corruption for the entries array to consume an empty freemap because
> > the nameval list (i.e. the firstused pointer in the xattr header)
> > starts beyond the end of the corrupted freemap.
> > 
> > Update the freemap size modification to account for the fact that
> > the freemap entry can be empty and thus stale.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> Hm.  freemap.size == 0 means the freemap entry is stale and therefore
> anything looking for free space in the leaf will ignore the entry, right?
> 

Yep, at least that's my understanding from the code in the caller that
explicitly checks for and skips freemaps where size == 0.

> If so,
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> 

Thanks!

Brian

> (Urk, there are still a lot of ASSERT-on-metadata in the dir/attr
> code...)
> 
> --D
> 
> > ---
> >  fs/xfs/libxfs/xfs_attr_leaf.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
> > index 85ec5945d29f..86155260d8b9 100644
> > --- a/fs/xfs/libxfs/xfs_attr_leaf.c
> > +++ b/fs/xfs/libxfs/xfs_attr_leaf.c
> > @@ -1510,7 +1510,9 @@ xfs_attr3_leaf_add_work(
> >  	for (i = 0; i < XFS_ATTR_LEAF_MAPSIZE; i++) {
> >  		if (ichdr->freemap[i].base == tmp) {
> >  			ichdr->freemap[i].base += sizeof(xfs_attr_leaf_entry_t);
> > -			ichdr->freemap[i].size -= sizeof(xfs_attr_leaf_entry_t);
> > +			ichdr->freemap[i].size -=
> > +				min_t(uint16_t, ichdr->freemap[i].size,
> > +						sizeof(xfs_attr_leaf_entry_t));
> >  		}
> >  	}
> >  	ichdr->usedbytes += xfs_attr_leaf_entsize(leaf, args->index);
> > -- 
> > 2.20.1
> > 
>