selinux: refactor changing booleans

Refactor the logic for changing SELinux policy booleans in a similar
manner to the refactoring of policy load, thereby reducing the
size of the critical section when the policy write-lock is held
and making it easier to convert the policy rwlock to RCU in the
future.  Instead of directly modifying the policydb in place, modify
a copy and then swap it into place through a single pointer update.
Only fully copy the portions of the policydb that are affected by
boolean changes to avoid the full cost of a deep policydb copy.
Introduce another level of indirection for the sidtab since changing
booleans does not require updating the sidtab, unlike policy load.
While we are here, create a common helper for notifying
other kernel components and userspace of a policy change and call it
from both security_set_bools() and selinux_policy_commit().

Based on an old (2004) patch by Kaigai Kohei [1] to convert the policy
rwlock to RCU that was deferred at the time since it did not
significantly improve performance and introduced complexity. Peter
Enderborg later submitted a patch series to convert to RCU [2] that
would have made changing booleans a much more expensive operation
by requiring a full policydb_write();policydb_read(); sequence to
deep copy the entire policydb and also had concerns regarding
atomic allocations.

This change is now simplified by the earlier work to encapsulate
policy state in the selinux_policy struct and to refactor
policy load.  After this change, the last major obstacle to
converting the policy rwlock to RCU is likely the sidtab live
convert support.

[1] https://lore.kernel.org/selinux/6e2f9128-e191-ebb3-0e87-74bfccb0767f@tycho.nsa.gov/
[2] https://lore.kernel.org/selinux/20180530141104.28569-1-peter.enderborg@sony.com/

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/security/selinux/ss/hashtab.c b/security/selinux/ss/hashtab.c
index d9287bb4bfeb..dab8c25c739b 100644
--- a/security/selinux/ss/hashtab.c
+++ b/security/selinux/ss/hashtab.c
@@ -122,6 +122,59 @@ void hashtab_stat(struct hashtab *h, struct hashtab_info *info)
 	info->max_chain_len = max_chain_len;
 }
 
+int hashtab_duplicate(struct hashtab *new, struct hashtab *orig,
+		int (*copy)(struct hashtab_node *new,
+			struct hashtab_node *orig, void *args),
+		int (*destroy)(void *k, void *d, void *args),
+		void *args)
+{
+	struct hashtab_node *cur, *tmp, *tail;
+	int i, rc;
+
+	memset(new, 0, sizeof(*new));
+
+	new->htable = kcalloc(orig->size, sizeof(*new->htable), GFP_KERNEL);
+	if (!new->htable)
+		return -ENOMEM;
+
+	new->size = orig->size;
+
+	for (i = 0; i < orig->size; i++) {
+		tail = NULL;
+		for (cur = orig->htable[i]; cur; cur = cur->next) {
+			tmp = kmem_cache_zalloc(hashtab_node_cachep,
+						GFP_KERNEL);
+			if (!tmp)
+				goto error;
+			rc = copy(tmp, cur, args);
+			if (rc) {
+				kmem_cache_free(hashtab_node_cachep, tmp);
+				goto error;
+			}
+			tmp->next = NULL;
+			if (!tail)
+				new->htable[i] = tmp;
+			else
+				tail->next = tmp;
+			tail = tmp;
+			new->nel++;
+		}
+	}
+
+	return 0;
+
+ error:
+	for (i = 0; i < new->size; i++) {
+		for (cur = new->htable[i]; cur; cur = tmp) {
+			tmp = cur->next;
+			destroy(cur->key, cur->datum, args);
+			kmem_cache_free(hashtab_node_cachep, cur);
+		}
+	}
+	kmem_cache_free(hashtab_node_cachep, new);
+	return -ENOMEM;
+}
+
 void __init hashtab_cache_init(void)
 {
 		hashtab_node_cachep = kmem_cache_create("hashtab_node",