From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756473AbaCZXSp (ORCPT ); Wed, 26 Mar 2014 19:18:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47087 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751683AbaCZXSo (ORCPT ); Wed, 26 Mar 2014 19:18:44 -0400 Message-ID: <53336049.4060901@redhat.com> Date: Thu, 27 Mar 2014 00:18:33 +0100 From: Daniel Borkmann User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 MIME-Version: 1.0 To: Sasha Levin CC: davem@davemloft.net, hannes@stressinduktion.org, tytso@mit.edu, linux-kernel@vger.kernel.org, eric.dumazet@gmail.com Subject: Re: [PATCH] random32: avoid attempt to late reseed if in the middle of seeding References: <1395853958-5083-1-git-send-email-sasha.levin@oracle.com> In-Reply-To: <1395853958-5083-1-git-send-email-sasha.levin@oracle.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/26/2014 06:12 PM, Sasha Levin wrote: > Commit 4af712e8df ("random32: add prandom_reseed_late() and call when > nonblocking pool becomes initialized") has added a late reseed stage > that happens as soon as the nonblocking pool is marked as initialized. > > This fails in the case that the nonblocking pool gets initialized > during __prandom_reseed()'s call to get_random_bytes(). In that case > we'd double back into __prandom_reseed() in an attempt to do a late > reseed - deadlocking on 'lock' early on in the boot process. > > Instead, just avoid even waiting to do a reseed if a reseed is already > occuring. > > Signed-off-by: Sasha Levin Thanks for catching! (If you want Dave to pick it up, please also Cc netdev.) Why not via spin_trylock_irqsave() ? Thus, if we already hold the lock, we do not bother any longer with doing the same work twice and just return. I.e. like: static void __prandom_reseed(bool late) { int i; unsigned long flags; static bool latch = false; static DEFINE_SPINLOCK(lock); /* Asking for random bytes might result in bytes getting * moved into the nonblocking pool and thus marking it * as initialized. In this case we would double back into * this function and attempt to do a late reseed. * Ignore the pointless attempt to reseed again if we're * already waiting for bytes when the nonblocking pool * got initialized. */ /* only allow initial seeding (late == false) once */ if (!spin_trylock_irqsave(&lock, flags)) return; if (latch && !late) goto out; latch = true; for_each_possible_cpu(i) { struct rnd_state *state = &per_cpu(net_rand_state,i); u32 seeds[4]; get_random_bytes(&seeds, sizeof(seeds)); state->s1 = __seed(seeds[0], 2U); state->s2 = __seed(seeds[1], 8U); state->s3 = __seed(seeds[2], 16U); state->s4 = __seed(seeds[3], 128U); prandom_warmup(state); } out: spin_unlock_irqrestore(&lock, flags); } > --- > lib/random32.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/lib/random32.c b/lib/random32.c > index 1e5b2df..b59da12 100644 > --- a/lib/random32.c > +++ b/lib/random32.c > @@ -241,14 +241,27 @@ static void __prandom_reseed(bool late) > { > int i; > unsigned long flags; > - static bool latch = false; > + static bool latch = false, reseeding = false; > static DEFINE_SPINLOCK(lock); > > + /* > + * Asking for random bytes might result in bytes getting > + * moved into the nonblocking pool and thus marking it > + * as initialized. In this case we would double back into > + * this function and attempt to do a late reseed. > + * Ignore the pointless attempt to reseed again if we're > + * already waiting for bytes when the nonblocking pool > + * got initialized > + */ > + if (reseeding) > + return; > + > /* only allow initial seeding (late == false) once */ > spin_lock_irqsave(&lock, flags); > if (latch && !late) > goto out; > latch = true; > + reseeding = true; > > for_each_possible_cpu(i) { > struct rnd_state *state = &per_cpu(net_rand_state,i); > @@ -263,6 +276,7 @@ static void __prandom_reseed(bool late) > prandom_warmup(state); > } > out: > + reseeding = false; > spin_unlock_irqrestore(&lock, flags); > } > >