Commit 89f39af1 authored Sep 26, 2016 by Oleg Nesterov Committed by Al Viro Oct 14, 2016

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>

parent 655042cc

fs/super.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -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;
		}

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