Re: [PATCH] xfs: libxfs: move xfs_perag_put late

[Carlos Maiolino]

On Sat, Nov 24, 2018 at 05:44:20PM +0800, Pan Bian wrote:
> The function xfs_alloc_get_freelist calls xfs_perag_put to drop the
> reference. In this case, pag may be released. However,
> pag->pagf_btreeblks is read and write after the put operation. This may
> result in a use-after-free bug. This patch moves the put operation late.
> 

The patch looks reasonable, can you detail more how did you find it? Via code
inspection of you hit this user-after-free in some way?

Cheers

> Signed-off-by: Pan Bian <bianpan2016@xxxxxxx>
> ---
>  fs/xfs/libxfs/xfs_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index e1c0c0d..4be387d 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2435,7 +2435,6 @@ xfs_alloc_get_freelist(
>  	be32_add_cpu(&agf->agf_flcount, -1);
>  	xfs_trans_agflist_delta(tp, -1);
>  	pag->pagf_flcount--;
> -	xfs_perag_put(pag);
>  
>  	logflags = XFS_AGF_FLFIRST | XFS_AGF_FLCOUNT;
>  	if (btreeblk) {
> @@ -2443,6 +2442,7 @@ xfs_alloc_get_freelist(
>  		pag->pagf_btreeblks++;
>  		logflags |= XFS_AGF_BTREEBLKS;
>  	}
> +	xfs_perag_put(pag);
>  
>  	xfs_alloc_log_agf(tp, agbp, logflags);
>  	*bnop = bno;
> -- 
> 2.7.4
> 
> 

-- 
Carlos