linux-kernel - Re: [PATCH] random: silence compiler warnings and fix race

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Date:   Mon, 19 Jun 2017 09:45:53 +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 2017-06-17 02:39:40 [+0200], Jason A. Donenfeld wrote:
> On Fri, Jun 16, 2017 at 4:35 PM, Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> > I wouldn't just push the lock one up as is but move that write part to
> > crng_init to remain within the locked section. Like that:
> 
> We can't quite do that, because invalidate_batched_entropy() needs to
> be called _before_ crng_init. Otherwise a concurrent call to
> get_random_u32/u64() will have crng_init being the wrong value when
> the batched entropy is still old.

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…

> 
> > 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().

> Jason

Sebastian