linux-kernel - [ 80/95] drivers/char/random.c: fix boot id uniqueness race

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Date:	Sun, 09 Sep 2012 23:43:10 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Theodore Tso" <tytso@....edu>, Matt Mackall <mpm@...enic.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Greg Kroah-Hartman <greg@...ah.com>
Subject: [ 80/95] drivers/char/random.c: fix boot id uniqueness race

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

commit 44e4360fa3384850d65dd36fb4e6e5f2f112709b upstream.

/proc/sys/kernel/random/boot_id can be read concurrently by userspace
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.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: "Theodore Ts'o" <tytso@....edu>
Cc: Matt Mackall <mpm@...enic.com>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Cc: Greg Kroah-Hartman <greg@...ah.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/char/random.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 54ca8b2..4ec04a7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1260,10 +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);
 


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