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Message-ID: <20170620064438.fcfnyklbhf7pqrc3@linutronix.de>
Date: Tue, 20 Jun 2017 08:44:38 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Theodore Ts'o <tytso@....edu>,
Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
kernel-hardening@...ts.openwall.com,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Eric Biggers <ebiggers3@...il.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Miller <davem@...emloft.net>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] random: silence compiler warnings and fix race
On 2017-06-19 22:55:37 [+0200], Jason A. Donenfeld wrote:
> On Mon, Jun 19, 2017 at 9:45 AM, Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> > ehm. You sure? I simply delayed the lock-dropping _after_ the state
> > variable was been modified. So it was basically what your patch did
> > except it was unlocked later…
>
> Yes, I'm sure. You moved the call to invalidate_batched_entropy() to
> be after the assignment of crng_init. However, the call to
> invalidate_batched_entropy() must be made _before_ the assignment of
> crng_init.
so you need to find a another way then. Doing the assignment after
dropping the lock opens another race.
> >> > Are use about that? I am not sure that the gcc will inline "crng_init"
> >> > read twice. It is not a local variable. READ_ONCE() is usually used
> >> > where gcc could cache a memory access but you do not want this. But hey!
> >> > If someone knows better I am here to learn.
> >>
> >> The whole purpose is that I _want_ it to cache the memory access so
> >> that it is _not_ inlined. So, based on your understanding, it does
> >> exactly what I intended it to do. The reason is that I'd like to avoid
> >> a lock imbalance, which could happen if the read is inlined.
> >
> > So it was good as it was which means you can drop that READ_ONCE().
>
> Except READ_ONCE ensures that the compiler will never inline it, so it
> actually needs to stay.
I don't think the compiler is allowed to inline it the way you describe
it. This would render any assignment to local variable useless. Also
the READ_ONCE creates worse code in this case (because the read can not
be delayed).
Sebastian
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