apparmor: add a cache entry expiration time aging out capability audit cache

When auditing capabilities, AppArmor uses a per-CPU, per-profile cache
such that the same capability for the same profile doesn't get repeatedly
audited, with the original goal of reducing audit logspam. However, this
cache does not have an expiration time, resulting in confusion when a
profile is shared across binaries (for example) and an expected DENIED
audit entry doesn't appear, despite the cache entry having been populated
much longer ago. This confusion was exacerbated by the per-CPU nature of
the cache resulting in the expected entries sporadically appearing when
the later denial+audit occurred on a different CPU.

To resolve this, record the last time a capability was audited for a
profile and add a timestamp expiration check before doing the audit.

v1 -> v2:
 - Hardcode a longer timeout and drop the patches making it a sysctl,
   after discussion with John Johansen.
 - Cache the expiration time instead of the last-audited time. This value
   can never be zero, which lets us drop the kernel_cap_t caps field from
   the cache struct.

Signed-off-by: Ryan Lee <[email protected]>
Signed-off-by: John Johansen <[email protected]>

Author: rlee287

security/apparmor/capability.c

@@ -12,6 +12,7 @@
 #include <linux/errno.h>
 #include <linux/gfp.h>
 #include <linux/security.h>
+#include <linux/timekeeping.h>
 
 #include "include/apparmor.h"
 #include "include/capability.h"
@@ -31,7 +32,8 @@ struct aa_sfs_entry aa_sfs_entry_caps[] = {
 
 struct audit_cache {
 	struct aa_profile *profile;
-	kernel_cap_t caps;
+	/* Capabilities go from 0 to CAP_LAST_CAP */
+	u64 ktime_ns_expiration[CAP_LAST_CAP+1];
 };
 
 static DEFINE_PER_CPU(struct audit_cache, audit_cache);
@@ -64,6 +66,8 @@ static void audit_cb(struct audit_buffer *ab, void *va)
 static int audit_caps(struct apparmor_audit_data *ad, struct aa_profile *profile,
 		      int cap, int error)
 {
+	const u64 AUDIT_CACHE_TIMEOUT_NS = 1000*1000*1000; /* 1 second */
+
 	struct aa_ruleset *rules = list_first_entry(&profile->rules,
 						    typeof(*rules), list);
 	struct audit_cache *ent;
@@ -89,7 +93,8 @@ static int audit_caps(struct apparmor_audit_data *ad, struct aa_profile *profile
 
 	/* Do simple duplicate message elimination */
 	ent = &get_cpu_var(audit_cache);
-	if (profile == ent->profile && cap_raised(ent->caps, cap)) {
+	/* If the capability was never raised the timestamp check would also catch that */
+	if (profile == ent->profile && ktime_get_ns() <= ent->ktime_ns_expiration[cap]) {
 		put_cpu_var(audit_cache);
 		if (COMPLAIN_MODE(profile))
 			return complain_error(error);
@@ -99,7 +104,7 @@ static int audit_caps(struct apparmor_audit_data *ad, struct aa_profile *profile
 		if (profile != ent->profile)
 			cap_clear(ent->caps);
 		ent->profile = aa_get_profile(profile);
-		cap_raise(ent->caps, cap);
+		ent->ktime_ns_expiration[cap] = ktime_get_ns() + AUDIT_CACHE_TIMEOUT_NS;
 	}
 	put_cpu_var(audit_cache);