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Date: Fri, 31 May 2019 13:02:15 +0100
From: David Howells <dhowells@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: dhowells@...hat.com, Jann Horn <jannh@...gle.com>,
    Greg KH <gregkh@...uxfoundation.org>,
    Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
    linux-fsdevel <linux-fsdevel@...r.kernel.org>,
    Linux API <linux-api@...r.kernel.org>, linux-block@...r.kernel.org,
    keyrings@...r.kernel.org,
    linux-security-module <linux-security-module@...r.kernel.org>,
    kernel list <linux-kernel@...r.kernel.org>,
    Kees Cook <keescook@...omium.org>,
    Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

Peter Zijlstra <peterz@...radead.org> wrote:

> Can you re-iterate the exact problem? I konw we talked about this in the
> past, but I seem to have misplaced those memories :/

Take this for example:

	void afs_put_call(struct afs_call *call)
	{
		struct afs_net *net = call->net;
		int n = atomic_dec_return(&call->usage);
		int o = atomic_read(&net->nr_outstanding_calls);

		trace_afs_call(call, afs_call_trace_put, n + 1, o,
			       __builtin_return_address(0));

		ASSERTCMP(n, >=, 0);
		if (n == 0) {
			...
		}
	}

I am printing the usage count in the afs_call tracepoint so that I can use it
to debug refcount bugs.  If I do it like this:

	void afs_put_call(struct afs_call *call)
	{
		int n = refcount_read(&call->usage);
		int o = atomic_read(&net->nr_outstanding_calls);

		trace_afs_call(call, afs_call_trace_put, n, o,
			       __builtin_return_address(0));

		if (refcount_dec_and_test(&call->usage)) {
			...
		}
	}

then there's a temporal gap between the usage count being read and the actual
atomic decrement in which another CPU can alter the count.  This can be
exacerbated by an interrupt occurring, a softirq occurring or someone enabling
the tracepoint.

I can't do the tracepoint after the decrement if refcount_dec_and_test()
returns false unless I save all the values from the object that I might need
as the object could be destroyed any time from that point on.  In this
particular case, that's just call->debug_id, but it could be other things in
other cases.

Note that I also can't touch the afs_net object in that situation either, and
the outstanding calls count that I record will potentially be out of date -
but there's not a lot I can do about that.

David

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