linux-ext4 - Re: [PATCH] ext4: Fix data races when using cached status extents

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Message-ID: <20230504105029.5sq7p6x3r6h7yjvw@quack3>
Date:   Thu, 4 May 2023 12:50:29 +0200
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+4a03518df1e31b537066@...kaller.appspotmail.com
Subject: Re: [PATCH] ext4: Fix data races when using cached status extents

On Wed 03-05-23 22:41:57, Theodore Ts'o wrote:
> On Wed, May 03, 2023 at 08:21:28PM +0200, Jan Kara wrote:
> > When using cached extent stored in extent status tree in tree->cache_es
> > another process holding ei->i_es_lock for reading can be racing with us
> > setting new value of tree->cache_es. If the compiler would decide to
> > refetch tree->cache_es at an unfortunate moment, it could result in a
> > bogus in_range() check. Fix the possible race by using READ_ONCE() when
> > using tree->cache_es only under ei->i_es_lock for reading.
> > 
> > Reported-by: syzbot+4a03518df1e31b537066@...kaller.appspotmail.com
> > Link: https://lore.kernel.org/all/000000000000d3b33905fa0fd4a6@google.com
> > Suggested-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Signed-off-by: Jan Kara <jack@...e.cz>
> 
> Don't we also need a WRITE_ONCE here?

Right, we should do that as well. I'll update the patch.

								Honza

> 
> diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> index 7bc221038c6c..4694582cf255 100644
> --- a/fs/ext4/extents_status.c
> +++ b/fs/ext4/extents_status.c
> @@ -293,7 +293,7 @@ static void __es_find_extent_range(struct inode *inode,
>  	}
>  
>  	if (es1 && matching_fn(es1)) {
> -		tree->cache_es = es1;
> +		WRITE_ONCE(tree->cache_es, es1);
>  		es->es_lblk = es1->es_lblk;
>  		es->es_len = es1->es_len;
>  		es->es_pblk = es1->es_pblk;
> 
> 					- Ted
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR