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

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Date:   Wed, 3 May 2023 22:41:57 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     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, 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?

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