linux-kernel - [PATCH 4.8 091/140] fs/super.c: fix race between freeze_super() and thaw

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20161026122224.270798201@linuxfoundation.org>
Date:   Wed, 26 Oct 2016 14:22:31 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: [PATCH 4.8 091/140] fs/super.c: fix race between freeze_super() and thaw_super()

4.8-stable 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: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/super.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/super.c
+++ b/fs/super.c
@@ -1379,8 +1379,8 @@ 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);
@@ -1399,7 +1399,7 @@ 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;
 	}