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, 6 insertions, 0 deletions

diff --git a/security/selinux/ss/hashtab.h b/security/selinux/ss/hashtab.h
index 3c952f0f01f9..043a773bf0b7 100644
--- a/security/selinux/ss/hashtab.h
+++ b/security/selinux/ss/hashtab.h
@@ -136,6 +136,12 @@ int hashtab_map(struct hashtab *h,
 		int (*apply)(void *k, void *d, void *args),
 		void *args);
 
+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);
+
 /* Fill info with some hash table statistics */
 void hashtab_stat(struct hashtab *h, struct hashtab_info *info);