selinux: cache the SID -> context string translation

Translating a context struct to string can be quite slow, especially if
the context has a lot of category bits set. This can cause quite
noticeable performance impact in situations where the translation needs
to be done repeatedly. A common example is a UNIX datagram socket with
the SO_PASSSEC option enabled, which is used e.g. by systemd-journald
when receiving log messages via datagram socket. This scenario can be
reproduced with:

    cat /dev/urandom | base64 | logger &
    timeout 30s perf record -p $(pidof systemd-journald) -a -g
    kill %1
    perf report -g none --pretty raw | grep security_secid_to_secctx

Before the caching introduced by this patch, computing the context
string (security_secid_to_secctx() function) takes up ~65% of
systemd-journald's CPU time (assuming a context with 1024 categories
set and Fedora x86_64 release kernel configs). After this patch
(assuming near-perfect cache hit ratio) this overhead is reduced to just
~2%.

This patch addresses the issue by caching a certain number (compile-time
configurable) of recently used context strings to speed up repeated
translations of the same context, while using only a small amount of
memory.

The cache is integrated into the existing sidtab table by adding a field
to each entry, which when not NULL contains an RCU-protected pointer to
a cache entry containing the cached string. The cache entries are kept
in a linked list sorted according to how recently they were used. On a
cache miss when the cache is full, the least recently used entry is
removed to make space for the new entry.

The patch migrates security_sid_to_context_core() to use the cache (also
a few other functions where it was possible without too much fuss, but
these mostly use the translation for logging in case of error, which is
rare).

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1733259
Cc: Michal Sekletar <msekleta@redhat.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Tested-by: Stephen Smalley <sds@tycho.nsa.gov>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
[PM: lots of merge fixups due to collisions with other sidtab patches]
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 84 insertions, 54 deletions

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 38fb6fdd65ca..743b85ede4ef 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -91,6 +91,12 @@ static int context_struct_to_string(struct policydb *policydb,
 				    char **scontext,
 				    u32 *scontext_len);
 
+static int sidtab_entry_to_string(struct policydb *policydb,
+				  struct sidtab *sidtab,
+				  struct sidtab_entry *entry,
+				  char **scontext,
+				  u32 *scontext_len);
+
 static void context_struct_compute_av(struct policydb *policydb,
 				      struct context *scontext,
 				      struct context *tcontext,
@@ -716,20 +722,21 @@ static void context_struct_compute_av(struct policydb *policydb,
 }
 
 static int security_validtrans_handle_fail(struct selinux_state *state,
-					   struct context *ocontext,
-					   struct context *ncontext,
-					   struct context *tcontext,
+					   struct sidtab_entry *oentry,
+					   struct sidtab_entry *nentry,
+					   struct sidtab_entry *tentry,
 					   u16 tclass)
 {
 	struct policydb *p = &state->ss->policydb;
+	struct sidtab *sidtab = state->ss->sidtab;
 	char *o = NULL, *n = NULL, *t = NULL;
 	u32 olen, nlen, tlen;
 
-	if (context_struct_to_string(p, ocontext, &o, &olen))
+	if (sidtab_entry_to_string(p, sidtab, oentry, &o, &olen))
 		goto out;
-	if (context_struct_to_string(p, ncontext, &n, &nlen))
+	if (sidtab_entry_to_string(p, sidtab, nentry, &n, &nlen))
 		goto out;
-	if (context_struct_to_string(p, tcontext, &t, &tlen))
+	if (sidtab_entry_to_string(p, sidtab, tentry, &t, &tlen))
 		goto out;
 	audit_log(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR,
 		  "op=security_validate_transition seresult=denied"
@@ -751,9 +758,9 @@ static int security_compute_validatetrans(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *ocontext;
-	struct context *ncontext;
-	struct context *tcontext;
+	struct sidtab_entry *oentry;
+	struct sidtab_entry *nentry;
+	struct sidtab_entry *tentry;
 	struct class_datum *tclass_datum;
 	struct constraint_node *constraint;
 	u16 tclass;
@@ -779,24 +786,24 @@ static int security_compute_validatetrans(struct selinux_state *state,
 	}
 	tclass_datum = policydb->class_val_to_struct[tclass - 1];
 
-	ocontext = sidtab_search(sidtab, oldsid);
-	if (!ocontext) {
+	oentry = sidtab_search_entry(sidtab, oldsid);
+	if (!oentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, oldsid);
 		rc = -EINVAL;
 		goto out;
 	}
 
-	ncontext = sidtab_search(sidtab, newsid);
-	if (!ncontext) {
+	nentry = sidtab_search_entry(sidtab, newsid);
+	if (!nentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, newsid);
 		rc = -EINVAL;
 		goto out;
 	}
 
-	tcontext = sidtab_search(sidtab, tasksid);
-	if (!tcontext) {
+	tentry = sidtab_search_entry(sidtab, tasksid);
+	if (!tentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, tasksid);
 		rc = -EINVAL;
@@ -805,15 +812,16 @@ static int security_compute_validatetrans(struct selinux_state *state,
 
 	constraint = tclass_datum->validatetrans;
 	while (constraint) {
-		if (!constraint_expr_eval(policydb, ocontext, ncontext,
-					  tcontext, constraint->expr)) {
+		if (!constraint_expr_eval(policydb, &oentry->context,
+					  &nentry->context, &tentry->context,
+					  constraint->expr)) {
 			if (user)
 				rc = -EPERM;
 			else
 				rc = security_validtrans_handle_fail(state,
-								     ocontext,
-								     ncontext,
-								     tcontext,
+								     oentry,
+								     nentry,
+								     tentry,
 								     tclass);
 			goto out;
 		}
@@ -855,7 +863,7 @@ int security_bounded_transition(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *old_context, *new_context;
+	struct sidtab_entry *old_entry, *new_entry;
 	struct type_datum *type;
 	int index;
 	int rc;
@@ -869,16 +877,16 @@ int security_bounded_transition(struct selinux_state *state,
 	sidtab = state->ss->sidtab;
 
 	rc = -EINVAL;
-	old_context = sidtab_search(sidtab, old_sid);
-	if (!old_context) {
+	old_entry = sidtab_search_entry(sidtab, old_sid);
+	if (!old_entry) {
 		pr_err("SELinux: %s: unrecognized SID %u\n",
 		       __func__, old_sid);
 		goto out;
 	}
 
 	rc = -EINVAL;
-	new_context = sidtab_search(sidtab, new_sid);
-	if (!new_context) {
+	new_entry = sidtab_search_entry(sidtab, new_sid);
+	if (!new_entry) {
 		pr_err("SELinux: %s: unrecognized SID %u\n",
 		       __func__, new_sid);
 		goto out;
@@ -886,10 +894,10 @@ int security_bounded_transition(struct selinux_state *state,
 
 	rc = 0;
 	/* type/domain unchanged */
-	if (old_context->type == new_context->type)
+	if (old_entry->context.type == new_entry->context.type)
 		goto out;
 
-	index = new_context->type;
+	index = new_entry->context.type;
 	while (true) {
 		type = policydb->type_val_to_struct[index - 1];
 		BUG_ON(!type);
@@ -901,7 +909,7 @@ int security_bounded_transition(struct selinux_state *state,
 
 		/* @newsid is bounded by @oldsid */
 		rc = 0;
-		if (type->bounds == old_context->type)
+		if (type->bounds == old_entry->context.type)
 			break;
 
 		index = type->bounds;
@@ -912,10 +920,10 @@ int security_bounded_transition(struct selinux_state *state,
 		char *new_name = NULL;
 		u32 length;
 
-		if (!context_struct_to_string(policydb, old_context,
-					      &old_name, &length) &&
-		    !context_struct_to_string(policydb, new_context,
-					      &new_name, &length)) {
+		if (!sidtab_entry_to_string(policydb, sidtab, old_entry,
+					    &old_name, &length) &&
+		    !sidtab_entry_to_string(policydb, sidtab, new_entry,
+					    &new_name, &length)) {
 			audit_log(audit_context(),
 				  GFP_ATOMIC, AUDIT_SELINUX_ERR,
 				  "op=security_bounded_transition "
@@ -1255,6 +1263,23 @@ static int context_struct_to_string(struct policydb *p,
 	return 0;
 }
 
+static int sidtab_entry_to_string(struct policydb *p,
+				  struct sidtab *sidtab,
+				  struct sidtab_entry *entry,
+				  char **scontext, u32 *scontext_len)
+{
+	int rc = sidtab_sid2str_get(sidtab, entry, scontext, scontext_len);
+
+	if (rc != -ENOENT)
+		return rc;
+
+	rc = context_struct_to_string(p, &entry->context, scontext,
+				      scontext_len);
+	if (!rc && scontext)
+		sidtab_sid2str_put(sidtab, entry, *scontext, *scontext_len);
+	return rc;
+}
+
 #include "initial_sid_to_string.h"
 
 int security_sidtab_hash_stats(struct selinux_state *state, char *page)
@@ -1282,7 +1307,7 @@ static int security_sid_to_context_core(struct selinux_state *state,
 {
 	struct policydb *policydb;
 	struct sidtab *sidtab;
-	struct context *context;
+	struct sidtab_entry *entry;
 	int rc = 0;
 
 	if (scontext)
@@ -1313,21 +1338,23 @@ static int security_sid_to_context_core(struct selinux_state *state,
 	read_lock(&state->ss->policy_rwlock);
 	policydb = &state->ss->policydb;
 	sidtab = state->ss->sidtab;
+
 	if (force)
-		context = sidtab_search_force(sidtab, sid);
+		entry = sidtab_search_entry_force(sidtab, sid);
 	else
-		context = sidtab_search(sidtab, sid);
-	if (!context) {
+		entry = sidtab_search_entry(sidtab, sid);
+	if (!entry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 			__func__, sid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
-	if (only_invalid && !context->len)
-		rc = 0;
-	else
-		rc = context_struct_to_string(policydb, context, scontext,
-					      scontext_len);
+	if (only_invalid && !entry->context.len)
+		goto out_unlock;
+
+	rc = sidtab_entry_to_string(policydb, sidtab, entry, scontext,
+				    scontext_len);
+
 out_unlock:
 	read_unlock(&state->ss->policy_rwlock);
 out:
@@ -1621,19 +1648,20 @@ int security_context_to_sid_force(struct selinux_state *state,
 
 static int compute_sid_handle_invalid_context(
 	struct selinux_state *state,
-	struct context *scontext,
-	struct context *tcontext,
+	struct sidtab_entry *sentry,
+	struct sidtab_entry *tentry,
 	u16 tclass,
 	struct context *newcontext)
 {
 	struct policydb *policydb = &state->ss->policydb;
+	struct sidtab *sidtab = state->ss->sidtab;
 	char *s = NULL, *t = NULL, *n = NULL;
 	u32 slen, tlen, nlen;
 	struct audit_buffer *ab;
 
-	if (context_struct_to_string(policydb, scontext, &s, &slen))
+	if (sidtab_entry_to_string(policydb, sidtab, sentry, &s, &slen))
 		goto out;
-	if (context_struct_to_string(policydb, tcontext, &t, &tlen))
+	if (sidtab_entry_to_string(policydb, sidtab, tentry, &t, &tlen))
 		goto out;
 	if (context_struct_to_string(policydb, newcontext, &n, &nlen))
 		goto out;
@@ -1692,7 +1720,8 @@ static int security_compute_sid(struct selinux_state *state,
 	struct policydb *policydb;
 	struct sidtab *sidtab;
 	struct class_datum *cladatum = NULL;
-	struct context *scontext = NULL, *tcontext = NULL, newcontext;
+	struct context *scontext, *tcontext, newcontext;
+	struct sidtab_entry *sentry, *tentry;
 	struct role_trans *roletr = NULL;
 	struct avtab_key avkey;
 	struct avtab_datum *avdatum;
@@ -1729,21 +1758,24 @@ static int security_compute_sid(struct selinux_state *state,
 	policydb = &state->ss->policydb;
 	sidtab = state->ss->sidtab;
 
-	scontext = sidtab_search(sidtab, ssid);
-	if (!scontext) {
+	sentry = sidtab_search_entry(sidtab, ssid);
+	if (!sentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 		       __func__, ssid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
-	tcontext = sidtab_search(sidtab, tsid);
-	if (!tcontext) {
+	tentry = sidtab_search_entry(sidtab, tsid);
+	if (!tentry) {
 		pr_err("SELinux: %s:  unrecognized SID %d\n",
 		       __func__, tsid);
 		rc = -EINVAL;
 		goto out_unlock;
 	}
 
+	scontext = &sentry->context;
+	tcontext = &tentry->context;
+
 	if (tclass && tclass <= policydb->p_classes.nprim)
 		cladatum = policydb->class_val_to_struct[tclass - 1];
 
@@ -1844,10 +1876,8 @@ static int security_compute_sid(struct selinux_state *state,
 
 	/* Check the validity of the context. */
 	if (!policydb_context_isvalid(policydb, &newcontext)) {
-		rc = compute_sid_handle_invalid_context(state, scontext,
-							tcontext,
-							tclass,
-							&newcontext);
+		rc = compute_sid_handle_invalid_context(state, sentry, tentry,
+							tclass, &newcontext);
 		if (rc)
 			goto out_unlock;
 	}