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Subject[PATCH] 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.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: "Theodore Ts'o" <tytso@mit.edu>
CC: Matt Mackall <mpm@selenic.com>
CC: Greg Kroah-Hartman <greg@kroah.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
@@ -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


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