linux-ext4 - Re: ext4: avoid drop reference to iloc.bh twice

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Date:   Thu, 18 Apr 2019 14:35:19 +0200
From:   Jan Kara <jack@...e.cz>
To:     Pan Bian <bianpan2016@....com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: ext4: avoid drop reference to iloc.bh twice

On Thu 18-04-19 16:31:18, Pan Bian wrote:
> The reference to iloc.bh has been dropped in ext4_mark_iloc_dirty.
> However, the reference is dropped again if error occurs during
> ext4_handle_dirty_metadata, which may result in use-after-free bugs.
> 
> Fixes: fb265c9cb49e("ext4: add ext4_sb_bread() to disambiguate ENOMEM
> cases")
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>

Thanks for the patch! Good spotting. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

I'm just wondering: Ted, shouldn't we make ext4_mark_iloc_dirty() clear the
iloc.bh pointer on it's own? I believe this is not the first time we had a
bug like this and it would also catch possible use-after-free issues in
case someone tried to use iloc.bh after the reference has been dropped.

Generally the scheme around ext4_get_inode_loc() and
ext4_mark_iloc_dirty() seems to be somewhat error prone. E.g. a quick audit
shows that there's bh leak in ext4_orphan_del() in one of the error paths.
I'll send patches.

								Honza

> ---
>  fs/ext4/resize.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index e7ae26e..4d5c0fc 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -874,6 +874,7 @@ static int add_new_gdb(handle_t *handle, struct inode *inode,
>  	err = ext4_handle_dirty_metadata(handle, NULL, gdb_bh);
>  	if (unlikely(err)) {
>  		ext4_std_error(sb, err);
> +		iloc.bh = NULL;
>  		goto errout;
>  	}
>  	brelse(dind);
> -- 
> 2.7.4
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR