fs/super.c: fix race between freeze_super() and thaw_super()

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@kyup.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/super.c b/fs/super.c
index c2ff475..47d11e0 100644 (file)
--- 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;
        }