[PATCH v2] ext4: Fix data races when using cached status extents

[PATCH v2] ext4: Fix data races when using cached status extents

[Jan Kara]

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@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000d3b33905fa0fd4a6@google.com
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/extents_status.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index 7bc221038c6c..595abb9e7d74 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -267,14 +267,12 @@ static void __es_find_extent_range(struct inode *inode,
 
 	/* see if the extent has been cached */
 	es->es_lblk = es->es_len = es->es_pblk = 0;
-	if (tree->cache_es) {
-		es1 = tree->cache_es;
-		if (in_range(lblk, es1->es_lblk, es1->es_len)) {
-			es_debug("%u cached by [%u/%u) %llu %x\n",
-				 lblk, es1->es_lblk, es1->es_len,
-				 ext4_es_pblock(es1), ext4_es_status(es1));
-			goto out;
-		}
+	es1 = READ_ONCE(tree->cache_es);
+	if (es1 && in_range(lblk, es1->es_lblk, es1->es_len)) {
+		es_debug("%u cached by [%u/%u) %llu %x\n",
+			 lblk, es1->es_lblk, es1->es_len,
+			 ext4_es_pblock(es1), ext4_es_status(es1));
+		goto out;
 	}
 
 	es1 = __es_tree_search(&tree->root, lblk);
@@ -293,7 +291,7 @@ 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;
@@ -931,14 +929,12 @@ int ext4_es_lookup_extent(struct inode *inode, ext4_lblk_t lblk,
 
 	/* find extent in cache firstly */
 	es->es_lblk = es->es_len = es->es_pblk = 0;
-	if (tree->cache_es) {
-		es1 = tree->cache_es;
-		if (in_range(lblk, es1->es_lblk, es1->es_len)) {
-			es_debug("%u cached by [%u/%u)\n",
-				 lblk, es1->es_lblk, es1->es_len);
-			found = 1;
-			goto out;
-		}
+	es1 = READ_ONCE(tree->cache_es);
+	if (es1 && in_range(lblk, es1->es_lblk, es1->es_len)) {
+		es_debug("%u cached by [%u/%u)\n",
+			 lblk, es1->es_lblk, es1->es_len);
+		found = 1;
+		goto out;
 	}
 
 	node = tree->root.rb_node;
-- 
2.35.3

Re: [PATCH v2] ext4: Fix data races when using cached status extents

[Theodore Ts'o]

On Thu, 04 May 2023 14:55:24 +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.
> 
> [...]

Applied, thanks!

[1/1] ext4: Fix data races when using cached status extents
      commit: 185e33d15ebf4a9d779fa78249b6cc95a071967e

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>