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Date:   Wed, 14 Apr 2021 21:47:35 +0800
From:   Zhang Yi <>
To:     <>
CC:     <>, <>,
        <>, <>, <>,
Subject: [RFC PATCH v2 5/7] ext4: use RCU to protect accessing superblock in blkdev_releasepage()

In blkdev_releasepage() we access the superblock structure directly, it
could be raced by umount filesystem on destroy superblock in put_super(),
and end up triggering a use after free issue.

drop cache                  umount filesystem
 get superblock
                               put_super() and free sb by destroy_work
 access superblock  <-- trigger use after free

This issue doesn't trigger easily in general because we get page locked
when invoking bdev_try_to_free_page(), and when umount filesystem the
procedure wait on page unlock, but it's not a guarantee. Fix this race
by use RCU to protect superblock in blkdev_releasepage().

Fixes: 87d8fe1ee6b8 ("add releasepage hooks to block devices which can be used by file systems")
Reported-by: Jan Kara <>
Signed-off-by: Zhang Yi <>
 fs/block_dev.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 5ed79a9063f6..cb84f347fb04 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1734,11 +1734,14 @@ EXPORT_SYMBOL_GPL(blkdev_read_iter);
 static int blkdev_releasepage(struct page *page, gfp_t wait)
-	struct super_block *super = BDEV_I(page->mapping->host)->bdev.bd_super;
+	struct super_block *super;
 	int ret = 0;
+	rcu_read_lock();
+	super = READ_ONCE(BDEV_I(page->mapping->host)->bdev.bd_super);
 	if (super && super->s_op->bdev_try_to_free_page)
 		ret = super->s_op->bdev_try_to_free_page(super, page, wait);
+	rcu_read_unlock();
 	if (!ret)
 		return try_to_free_buffers(page);
 	return 0;

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