[PATCH] keys: allocate key serial numbers randomly
Cause key_alloc_serial() to generate key serial numbers randomly rather than
in linear sequence.
Using an linear sequence permits a covert communication channel to be
established, in which one process can communicate with another by creating or
not creating new keys within a certain timeframe. The second process can
probe for the expected next key serial number and judge its existence by the
error returned.
This is a problem as the serial number namespace is globally shared between
all tasks, regardless of their context.
For more information on this topic, this old TCSEC guide is recommended:
http://www.radium.ncsc.mil/tpep/library/rainbow/NCSC-TG-030.html
Signed-off-by: Michael LeMay <mdlemay@epoch.ncsc.mil>
Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/security/keys/key.c b/security/keys/key.c
index 3601fdd..43295ca 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -15,11 +15,11 @@
#include <linux/slab.h>
#include <linux/security.h>
#include <linux/workqueue.h>
+#include <linux/random.h>
#include <linux/err.h>
#include "internal.h"
static kmem_cache_t *key_jar;
-static key_serial_t key_serial_next = 3;
struct rb_root key_serial_tree; /* tree of keys indexed by serial */
DEFINE_SPINLOCK(key_serial_lock);
@@ -169,22 +169,23 @@
/*****************************************************************************/
/*
* assign a key the next unique serial number
- * - we work through all the serial numbers between 2 and 2^31-1 in turn and
- * then wrap
+ * - these are assigned randomly to avoid security issues through covert
+ * channel problems
*/
static inline void key_alloc_serial(struct key *key)
{
struct rb_node *parent, **p;
struct key *xkey;
- spin_lock(&key_serial_lock);
-
- /* propose a likely serial number and look for a hole for it in the
+ /* propose a random serial number and look for a hole for it in the
* serial number tree */
- key->serial = key_serial_next;
- if (key->serial < 3)
- key->serial = 3;
- key_serial_next = key->serial + 1;
+ do {
+ get_random_bytes(&key->serial, sizeof(key->serial));
+
+ key->serial >>= 1; /* negative numbers are not permitted */
+ } while (key->serial < 3);
+
+ spin_lock(&key_serial_lock);
parent = NULL;
p = &key_serial_tree.rb_node;
@@ -204,12 +205,11 @@
/* we found a key with the proposed serial number - walk the tree from
* that point looking for the next unused serial number */
- serial_exists:
+serial_exists:
for (;;) {
- key->serial = key_serial_next;
+ key->serial++;
if (key->serial < 2)
key->serial = 2;
- key_serial_next = key->serial + 1;
if (!rb_parent(parent))
p = &key_serial_tree.rb_node;
@@ -228,7 +228,7 @@
}
/* we've found a suitable hole - arrange for this key to occupy it */
- insert_here:
+insert_here:
rb_link_node(&key->serial_node, parent, p);
rb_insert_color(&key->serial_node, &key_serial_tree);