linux-kernel - [PATCH] char random: fix boot id uniqueness race (v3)

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Date:	Wed, 15 Feb 2012 13:40:45 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Matt Mackall <mpm@...enic.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Greg Kroah-Hartman <greg@...ah.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] char random: fix boot id uniqueness race (v3)

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 spinlock.

Given that this operation is not frequently used, hitting the spinlock
on each call should not be an issue.

* Changelog since v1:
- boot_id_mutex is now declared within the proc_do_uuid scope.
- added explanation for memory barriers.

* Changelog since v2:
- simplify: use spinlock on all paths, suggested by Eric Dumazet.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: "Theodore Ts'o" <tytso@....edu>
CC: Matt Mackall <mpm@...enic.com>
CC: Eric Dumazet <eric.dumazet@...il.com>
CC: Greg Kroah-Hartman <greg@...ah.com>
---
 drivers/char/random.c |   11 ++++++++---
 1 file changed, 8 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
@@ -1250,10 +1250,15 @@ static int proc_do_uuid(ctl_table *table
 	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);
 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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