Re: [PATCH] xfs: fix use after free in buf log item unlock assert

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

On Wed, Feb 06, 2019 at 11:01:07AM -0500, Brian Foster wrote:
> The xfs_buf_log_item ->iop_unlock() callback asserts that the buffer
> is unlocked when either non-stale or aborted. This assert occurs
> after the bli refcount has been dropped and the log item potentially
> freed. The aborted check is thus a potential use after free. This
> problem has been reproduced with KASAN enabled via generic/475.
> 
> Fix up xfs_buf_item_unlock() to query aborted state before the bli
> reference is dropped to prevent a potential use after free.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

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

--D

> ---
>  fs/xfs/xfs_buf_item.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 010db5f8fb00..65b32acfa0f6 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -605,6 +605,8 @@ xfs_buf_item_unlock(
>  #if defined(DEBUG) || defined(XFS_WARN)
>  	bool			ordered = bip->bli_flags & XFS_BLI_ORDERED;
>  	bool			dirty = bip->bli_flags & XFS_BLI_DIRTY;
> +	bool			aborted = test_bit(XFS_LI_ABORTED,
> +						   &lip->li_flags);
>  #endif
>  
>  	trace_xfs_buf_item_unlock(bip);
> @@ -633,7 +635,7 @@ xfs_buf_item_unlock(
>  	released = xfs_buf_item_put(bip);
>  	if (hold || (stale && !released))
>  		return;
> -	ASSERT(!stale || test_bit(XFS_LI_ABORTED, &lip->li_flags));
> +	ASSERT(!stale || aborted);
>  	xfs_buf_relse(bp);
>  }
>  
> -- 
> 2.17.2
>