char random: fix boot id uniqueness race

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.

---
 drivers/char/random.c |   20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

Index: linux-2.6-lttng/drivers/char/random.c
===================================================================
@@ -1231,6 +1231,8 @@ static int min_read_thresh = 8, min_writ
 static int max_read_thresh = INPUT_POOL_WORDS * 32;
 static int max_write_thresh = INPUT_POOL_WORDS * 32;
 static char sysctl_bootid[16];
+static int boot_id_generated;
+static DEFINE_MUTEX(boot_id_mutex);
 
 /*
  * These functions is used to return both the bootid UUID, and random

@@ -1250,10 +1252,22 @@ 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 {
+		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);
 

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

Hi Ted,

No problem, can do.

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.

Please let me know if there are aspects of your question I can address better,

Thanks,

Mathieu

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