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Message-ID: <1329284916.2555.34.camel@edumazet-laptop>
Date: Wed, 15 Feb 2012 06:48:36 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org,
Matt Mackall <mpm@...enic.com>,
Greg Kroah-Hartman <greg@...ah.com>
Subject: Re: [PATCH] char random: fix boot id uniqueness race (v2)
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 <mathieu.desnoyers@...icios.com>
> CC: "Theodore Ts'o" <tytso@....edu>
> CC: Matt Mackall <mpm@...enic.com>
> CC: Greg Kroah-Hartman <greg@...ah.com>
> ---
> 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;
--
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