Re: [PATCH] xfs: reinit btree pointer on attr tree inactivation walk

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Oct 11, 2017 at 02:11:44PM -0300, Marco Benatto wrote:
> Hello all
> 
> On Fri, Oct 6, 2017 at 5:47 PM, Darrick J. Wong <darrick.wong@oracle.com> wrote:
> >
> > On Fri, Oct 06, 2017 at 04:07:40PM -0400, Brian Foster wrote:
> > > xfs_attr3_root_inactive() walks the attr fork tree to invalidate the
> > > associated blocks. xfs_attr3_node_inactive() recursively descends
> > > from internal blocks to leaf blocks, caching block address values
> > > along the way to revisit parent blocks, locate the next entry and
> > > descend down that branch of the tree.
> > >
> > > The code that attempts to reread the parent block is unsafe because
> > > it assumes that the local xfs_da_node_entry pointer remains valid
> > > after an xfs_trans_brelse() and re-read of the parent buffer. Under
> > > heavy memory pressure, it is possible that the buffer has been
> > > reclaimed and reallocated by the time the parent block is reread.
> > > This means that 'btree' can point to an invalid memory address, lead
> > > to a random/garbage value for child_fsb and cause the subsequent
> > > read of the attr fork to go off the rails and return a NULL buffer
> > > for an attr fork offset that is most likely not allocated.
> > >
> > > Note that this problem can be manufactured by setting
> > > XFS_ATTR_BTREE_REF to 0 to prevent LRU caching of attr buffers,
> > > creating a file with a multi-level attr fork and removing it to
> > > trigger inactivation.
> > >
> > > To address this problem, reinit the node/btree pointers to the
> > > parent buffer after it has been re-read. This ensures btree points
> > > to a valid record and allows the walk to proceed.
> > >
> > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> >
> > Looks ok,
> > Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> >
> > /me wonders if this is a good enough reason to introduce a new errortag
> > that turns xfs_buf_set_ref into a no-op and fills bp->b_addr with
> > garbage prior to releasing the memory to weed out any other dangling
> > pointers?
> >
> > > ---
> > >
> > > I suspect this is the cause of the NULL buf problem down in
> > > xfs_attr_inactive(). I can manufacture an instance of that problem as
> > > noted above. We have a customer who's hitting that problem and will
> > > attempt to validate this fix, but there is no confirmation as of yet.
> > > I'm posting this for review in the meantime because this seems like a
> > > legit fix regardless of whether they are hitting this or something else.
> >
> > Let me know what they report back.
> 
> Just to let you know, we've got some news regarding this testing and the
> patch seems effective to fix the issue they were facing before at
> xfs_attr_inactive() case.

Ok, I'll queue this up for the next update.  Thank you for confirming!

--D

> 
> >
> >
> > --D
> >
> > > Brian
> > >
> > >  fs/xfs/xfs_attr_inactive.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/fs/xfs/xfs_attr_inactive.c b/fs/xfs/xfs_attr_inactive.c
> > > index ebd66b1..e3a950e 100644
> > > --- a/fs/xfs/xfs_attr_inactive.c
> > > +++ b/fs/xfs/xfs_attr_inactive.c
> > > @@ -302,6 +302,8 @@ xfs_attr3_node_inactive(
> > >                                                &bp, XFS_ATTR_FORK);
> > >                       if (error)
> > >                               return error;
> > > +                     node = bp->b_addr;
> > > +                     btree = dp->d_ops->node_tree_p(node);
> > >                       child_fsb = be32_to_cpu(btree[i + 1].before);
> > >                       xfs_trans_brelse(*trans, bp);
> > >               }
> > > --
> > > 2.9.5
> > >
> > > --
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> 
> 
> Thanks,
> 
> 
> -- 
> Marco Benatto
> --
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