Re: [PATCH 1/4] xfs: fix brainos in the refcount scrubber's rmap fragment processor

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

On Thu, Nov 12, 2020 at 06:21:52PM +0530, Chandan Babu R wrote:
> On Monday 9 November 2020 11:47:39 PM IST Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Fix some serious WTF in the reference count scrubber's rmap fragment
> > processing.  The code comment says that this loop is supposed to move
> > all fragment records starting at or before bno onto the worklist, but
> > there's no obvious reason why nr (the number of items added) should
> > increment starting from 1, and breaking the loop when we've added the
> > target number seems dubious since we could have more rmap fragments that
> > should have been added to the worklist.
> > 
> > This seems to manifest in xfs/411 when adding one to the refcount field.
> > 
> > Fixes: dbde19da9637 ("xfs: cross-reference the rmapbt data with the refcountbt")
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/scrub/refcount.c |    8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> > 
> > 
> > diff --git a/fs/xfs/scrub/refcount.c b/fs/xfs/scrub/refcount.c
> > index beaeb6fa3119..dd672e6bbc75 100644
> > --- a/fs/xfs/scrub/refcount.c
> > +++ b/fs/xfs/scrub/refcount.c
> > @@ -170,7 +170,6 @@ xchk_refcountbt_process_rmap_fragments(
> >  	 */
> >  	INIT_LIST_HEAD(&worklist);
> >  	rbno = NULLAGBLOCK;
> > -	nr = 1;
> >  
> >  	/* Make sure the fragments actually /are/ in agbno order. */
> >  	bno = 0;
> > @@ -184,15 +183,14 @@ xchk_refcountbt_process_rmap_fragments(
> >  	 * Find all the rmaps that start at or before the refc extent,
> >  	 * and put them on the worklist.
> >  	 */
> > +	nr = 0;
> >  	list_for_each_entry_safe(frag, n, &refchk->fragments, list) {
> > -		if (frag->rm.rm_startblock > refchk->bno)
> > -			goto done;
> > +		if (frag->rm.rm_startblock > refchk->bno || nr > target_nr)
> > +			break;
> 
> In the case of fuzzed refcnt value of 1, The condition "nr > target_nr" causes
> "nr != target_nr" condition (appearing after the loop) to evaluate to true
> (since atleast two rmap entries would be present for the refcount extent)
> which in turn causes xchk_refcountbt_xref_rmap() to flag the data structure as
> corrupt. Please let me know if my understanding of the code flow is correct?

Right.

--D

> >  		bno = frag->rm.rm_startblock + frag->rm.rm_blockcount;
> >  		if (bno < rbno)
> >  			rbno = bno;
> >  		list_move_tail(&frag->list, &worklist);
> > -		if (nr == target_nr)
> > -			break;
> >  		nr++;
> >  	}
> >  
> > 
> > 
> 
> 
> -- 
> chandan
> 
> 
>