linux-kernel - [PATCH 3.16 121/306] fs/super.c: fix race between freeze_super() and thaw

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Message-ID: <lsq.1487198500.755947877@decadent.org.uk>
Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Al Viro" <viro@...iv.linux.org.uk>,
        "Oleg Nesterov" <oleg@...hat.com>
Subject: [PATCH 3.16 121/306] fs/super.c: fix race between freeze_super()
 and thaw_super()

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@...hat.com>

commit 89f39af129382a40d7cd1f6914617282cfeee28e upstream.

Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than
frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which
drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering.

In this case thaw_super() will wrongly call s_op->unfreeze_fs() before
it was actually frozen, and call sb_freeze_unlock() which leads to the
unbalanced percpu_up_write(). Unfortunately lockdep can't detect this,
so this triggers misc BUG_ON()'s in kernel/rcu/sync.c.

Reported-and-tested-by: Nikolay Borisov <kernel@...p.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/super.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/super.c
+++ b/fs/super.c
@@ -1337,8 +1337,8 @@ int freeze_super(struct super_block *sb)
 		}
 	}
 	/*
-	 * This is just for debugging purposes so that fs can warn if it
-	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
+	 * For debugging purposes so that fs can warn if it sees write activity
+	 * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
 	 */
 	sb->s_writers.frozen = SB_FREEZE_COMPLETE;
 	up_write(&sb->s_umount);
@@ -1357,7 +1357,7 @@ int thaw_super(struct super_block *sb)
 	int error;
 
 	down_write(&sb->s_umount);
-	if (sb->s_writers.frozen == SB_UNFROZEN) {
+	if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
 		up_write(&sb->s_umount);
 		return -EINVAL;
 	}