From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751512Ab2BOFsn (ORCPT ); Wed, 15 Feb 2012 00:48:43 -0500 Received: from mail-we0-f174.google.com ([74.125.82.174]:56820 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751118Ab2BOFsm (ORCPT ); Wed, 15 Feb 2012 00:48:42 -0500 Message-ID: <1329284916.2555.34.camel@edumazet-laptop> Subject: Re: [PATCH] char random: fix boot id uniqueness race (v2) From: Eric Dumazet To: Mathieu Desnoyers Cc: "Theodore Ts'o" , linux-kernel@vger.kernel.org, Matt Mackall , Greg Kroah-Hartman Date: Wed, 15 Feb 2012 06:48:36 +0100 In-Reply-To: <20120215041014.GA15846@Krystal> References: <20120215041014.GA15846@Krystal> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.2- Content-Transfer-Encoding: 8bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le mardi 14 février 2012 à 23:10 -0500, Mathieu Desnoyers a écrit : > The proc file /proc/sys/kernel/random/boot_id can be read concurrently > by user-space processes. If two (or more) user-space processes > concurrently read boot_id when sysctl_bootid is not yet assigned, a race > can occur making boot_id differ between the reads. Because the whole > point of the boot id is to be unique across a kernel execution, fix this > by protecting this operation with a mutex, and introduce a > boot_id_generated flag, along with appropriate memory barriers, to let > the fast-path know if the boot ID has been generated without having to > hold the mutex. > > I propose this approach rather than setting it up within an initcall(), > because letting execution randomness add to entropy before populating > the boot id seems to be a wanted property. Also, populating it lazily > rather than at boot time only makes the performance hit be taken when > boot_id is being read. > > > Q: Why are these memory barriers required ? Aren't the mutexes already > dealing with ordering ? > > The need for memory barriers is a consequence of letting the fast-path > run without holding this mutex. > > Here is the race dealt with by the smp_rmb()/smp_wmb(). I'm showing the > result of reversed write order here: > > CPU A CPU B > > Load boot_id_generated > (test -> false) > mutex_lock(&boot_id_mutex) > (implied memory barrier > with acquire semantic) > Load boot_id_generated again > (test -> false) > boot_id_generated = 1 > (both the compiler and > CPU are free to reorder > the boot_id_generated > store before uuid stores) > Load boot_id_generated > (test -> true) > Load uuid content > (races with generate_random_uuid: > result either 0 or corrupted) > Return corrupted uuid. > generate_random_uuid(uuid) > mutex_unlock(&boot_id_mutex) > > I prefer not requiring the fast-path to take a mutex, because this > would transform a read-mostly operation into an operation that > requires cache-line exchanges (the mutex). However, if we want the > fast-path to be mutex-free, we need to enforce order with > memory barriers: smp_rmb on the read-side, smp_wmb on the > update-side. Failure to do so leads to the race shown above, where > a corrupted boot_id can be returned. > > > * Changelog since v1: > - boot_id_mutex is now declared within the proc_do_uuid scope. > - added explanation for memory barriers. > > Signed-off-by: Mathieu Desnoyers > CC: "Theodore Ts'o" > CC: Matt Mackall > CC: Greg Kroah-Hartman > --- > drivers/char/random.c | 20 +++++++++++++++++--- > 1 file changed, 17 insertions(+), 3 deletions(-) > > Index: linux-2.6-lttng/drivers/char/random.c > =================================================================== > --- linux-2.6-lttng.orig/drivers/char/random.c > +++ linux-2.6-lttng/drivers/char/random.c > @@ -1244,16 +1244,30 @@ static char sysctl_bootid[16]; > static int proc_do_uuid(ctl_table *table, int write, > void __user *buffer, size_t *lenp, loff_t *ppos) > { > + static int boot_id_generated; > + static DEFINE_MUTEX(boot_id_mutex); > ctl_table fake_table; > unsigned char buf[64], tmp_uuid[16], *uuid; > > uuid = table->data; > if (!uuid) { > uuid = tmp_uuid; > - uuid[8] = 0; > - } > - if (uuid[8] == 0) > generate_random_uuid(uuid); > + } else { > + if (unlikely(!ACCESS_ONCE(boot_id_generated))) { > + mutex_lock(&boot_id_mutex); > + if (!boot_id_generated) { > + generate_random_uuid(uuid); > + /* Store uuid before boot_id_generated. */ > + smp_wmb(); > + boot_id_generated = 1; > + } > + mutex_unlock(&boot_id_mutex); > + } else { > + /* Load boot_id_generated before uuid */ > + smp_rmb(); > + } > + } > > sprintf(buf, "%pU", uuid); > > This seems overly complex to me. I doubt this is performance critical path ? What about a basic patch like : diff --git a/drivers/char/random.c b/drivers/char/random.c index 54ca8b2..af6040d 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1260,11 +1260,15 @@ static int proc_do_uuid(ctl_table *table, int write, uuid = table->data; if (!uuid) { uuid = tmp_uuid; - uuid[8] = 0; - } - if (uuid[8] == 0) generate_random_uuid(uuid); + } else { + static DEFINE_SPINLOCK(bootid_spinlock); + spin_lock(&bootid_spinlock); + if (!uuid[8]) + generate_random_uuid(uuid); + spin_unlock(&bootid_spinlock); + } sprintf(buf, "%pU", uuid); fake_table.data = buf;