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From: Zhang Yi <yi.zhang@huawei.com>
To: <linux-ext4@vger.kernel.org>
Cc: <linux-fsdevel@vger.kernel.org>, <tytso@mit.edu>,
	<adilger.kernel@dilger.ca>, <jack@suse.cz>, <yi.zhang@huawei.com>,
	<yukuai3@huawei.com>
Subject: [RFC PATCH v2 5/7] ext4: use RCU to protect accessing superblock in blkdev_releasepage()
Date: Wed, 14 Apr 2021 21:47:35 +0800
Message-ID: <20210414134737.2366971-6-yi.zhang@huawei.com> (raw)
In-Reply-To: <20210414134737.2366971-1-yi.zhang@huawei.com>

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
bdev_try_to_free_page()
 get superblock
                             deactivate_locked_super()
                               ...
                               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
kill_block_super()->..->kill_bdev()->truncate_inode_pages_range()
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 <jack@suse.cz>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
---
 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;
-- 
2.25.4


  parent reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-14 13:47 [RFC PATCH v2 0/7] ext4, jbd2: fix 3 issues about bdev_try_to_free_page() Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 1/7] jbd2: remove the out label in __jbd2_journal_remove_checkpoint() Zhang Yi
2021-04-21 10:01   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 2/7] jbd2: ensure abort the journal if detect IO error when writing original buffer back Zhang Yi
2021-04-21 13:20   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 3/7] jbd2: don't abort the journal when freeing buffers Zhang Yi
2021-04-21 13:23   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 4/7] jbd2: do not free buffers in jbd2_journal_try_to_free_buffers() Zhang Yi
2021-04-15 14:46   ` Christoph Hellwig
2021-04-14 13:47 ` Zhang Yi [this message]
2021-04-15 14:48   ` [RFC PATCH v2 5/7] ext4: use RCU to protect accessing superblock in blkdev_releasepage() Christoph Hellwig
2021-04-14 13:47 ` [RFC PATCH v2 6/7] fs: introduce a usage count into the superblock Zhang Yi
2021-04-15 14:40   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super() Zhang Yi
2021-04-15 14:52   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi
2021-04-20 13:08       ` Christoph Hellwig
2021-04-21 13:46         ` Jan Kara
2021-04-21 16:57           ` Theodore Ts'o
2021-04-22  9:04             ` Jan Kara
2021-04-23 11:39               ` Zhang Yi
2021-04-23 16:06                 ` Jan Kara
2021-04-23 14:40               ` Theodore Ts'o

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