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

[Chandan Babu R <chandanrlinux@gmail.com>]

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?

>  		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