[PATCH v3] libselinux: selinux_set_mapping: fix handling of unknown classes/perms

* [PATCH v3] libselinux: selinux_set_mapping: fix handling of unknown classes/perms
@ 2019-02-25 15:49 Stephen Smalley
  2019-03-01 11:53 ` Petr Lautrbach
  0 siblings, 1 reply; 2+ messages in thread
From: Stephen Smalley @ 2019-02-25 15:49 UTC (permalink / raw)
  To: paul; +Cc: dominick.grift, selinux, Stephen Smalley

The libselinux selinux_set_mapping() implementation was never updated
to handle unknown classes/permissions based on the policy handle_unknown
flag.  Update it and the internal mapping functions to gracefully
handle unknown classes/permissions.  Add a security_reject_unknown()
interface to expose the corresponding selinuxfs node and use it when
creating a mapping to decide whether to fail immediately or proceed.

This enables dbus-daemon and XSELinux, which use selinux_set_mapping(),
to continue working with the dummy policy or other policies that lack
their userspace class/permission definitions as long as the policy
was built with -U allow.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
---
v3 adjusts the map_decision() logic to use else if for the
allow_unknown test since we do not need to set the permission
more than once.

 libselinux/include/selinux/selinux.h |  5 +-
 libselinux/src/compute_av.c          | 14 ++++--
 libselinux/src/mapping.c             | 70 ++++++++++++++++++++++------
 libselinux/src/reject_unknown.c      | 40 ++++++++++++++++
 libselinux/src/selinux_internal.h    |  1 +
 5 files changed, 113 insertions(+), 17 deletions(-)
 create mode 100644 libselinux/src/reject_unknown.c

diff --git a/libselinux/include/selinux/selinux.h b/libselinux/include/selinux/selinux.h
index 01201eee..a34d54fc 100644
--- a/libselinux/include/selinux/selinux.h
+++ b/libselinux/include/selinux/selinux.h
@@ -328,7 +328,10 @@ extern int security_getenforce(void);
 /* Set the enforce flag value. */
 extern int security_setenforce(int value);
 
-/* Get the behavior for undefined classes/permissions */
+/* Get the load-time behavior for undefined classes/permissions */
+extern int security_reject_unknown(void);
+
+/* Get the runtime behavior for undefined classes/permissions */
 extern int security_deny_unknown(void);
 
 /* Get the checkreqprot value */
diff --git a/libselinux/src/compute_av.c b/libselinux/src/compute_av.c
index 1d05e7b6..a47cffe9 100644
--- a/libselinux/src/compute_av.c
+++ b/libselinux/src/compute_av.c
@@ -20,6 +20,7 @@ int security_compute_av_flags_raw(const char * scon,
 	char *buf;
 	size_t len;
 	int fd, ret;
+	security_class_t kclass;
 
 	if (!selinux_mnt) {
 		errno = ENOENT;
@@ -38,8 +39,9 @@ int security_compute_av_flags_raw(const char * scon,
 		goto out;
 	}
 
+	kclass = unmap_class(tclass);
 	snprintf(buf, len, "%s %s %hu %x", scon, tcon,
-		 unmap_class(tclass), unmap_perm(tclass, requested));
+		 kclass, unmap_perm(tclass, requested));
 
 	ret = write(fd, buf, strlen(buf));
 	if (ret < 0)
@@ -60,8 +62,14 @@ int security_compute_av_flags_raw(const char * scon,
 	} else if (ret < 6)
 		avd->flags = 0;
 
-	/* If tclass invalid, kernel sets avd according to deny_unknown flag */
-	if (tclass != 0)
+	/*
+	 * If the tclass could not be mapped to a kernel class at all, the
+	 * kernel will have already set avd according to the
+	 * handle_unknown flag and we do not need to do anything further.
+	 * Otherwise, we must map the permissions within the returned
+	 * avd to the userspace permission values.
+	 */
+	if (kclass != 0)
 		map_decision(tclass, avd);
 
 	ret = 0;
diff --git a/libselinux/src/mapping.c b/libselinux/src/mapping.c
index f205804b..33cea5ae 100644
--- a/libselinux/src/mapping.c
+++ b/libselinux/src/mapping.c
@@ -6,9 +6,12 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stdbool.h>
 #include <selinux/selinux.h>
 #include <selinux/avc.h>
+#include "callbacks.h"
 #include "mapping.h"
+#include "selinux_internal.h"
 
 /*
  * Class and permission mappings
@@ -33,6 +36,9 @@ selinux_set_mapping(struct security_class_mapping *map)
 	size_t size = sizeof(struct selinux_mapping);
 	security_class_t i, j;
 	unsigned k;
+	bool print_unknown_handle = false;
+	bool reject = (security_reject_unknown() == 1);
+	bool deny = (security_deny_unknown() == 1);
 
 	free(current_mapping);
 	current_mapping = NULL;
@@ -62,8 +68,16 @@ selinux_set_mapping(struct security_class_mapping *map)
 		struct selinux_mapping *p_out = current_mapping + j;
 
 		p_out->value = string_to_security_class(p_in->name);
-		if (!p_out->value)
-			goto err2;
+		if (!p_out->value) {
+			selinux_log(SELINUX_INFO,
+				    "SELinux: Class %s not defined in policy.\n",
+				    p_in->name);
+			if (reject)
+				goto err2;
+			p_out->num_perms = 0;
+			print_unknown_handle = true;
+			continue;
+		}
 
 		k = 0;
 		while (p_in->perms[k]) {
@@ -74,13 +88,24 @@ selinux_set_mapping(struct security_class_mapping *map)
 			}
 			p_out->perms[k] = string_to_av_perm(p_out->value,
 							    p_in->perms[k]);
-			if (!p_out->perms[k])
-				goto err2;
+			if (!p_out->perms[k]) {
+				selinux_log(SELINUX_INFO,
+					    "SELinux:  Permission %s in class %s not defined in policy.\n",
+					    p_in->perms[k], p_in->name);
+				if (reject)
+					goto err2;
+				print_unknown_handle = true;
+			}
 			k++;
 		}
 		p_out->num_perms = k;
 	}
 
+	if (print_unknown_handle)
+		selinux_log(SELINUX_INFO,
+			    "SELinux: the above unknown classes and permissions will be %s\n",
+			    deny ? "denied" : "allowed");
+
 	/* Set the mapping size here so the above lookups are "raw" */
 	current_mapping_size = i;
 	return 0;
@@ -184,27 +209,46 @@ void
 map_decision(security_class_t tclass, struct av_decision *avd)
 {
 	if (tclass < current_mapping_size) {
-		unsigned i;
+		bool allow_unknown = (security_deny_unknown() == 0);
+		struct selinux_mapping *mapping = &current_mapping[tclass];
+		unsigned int i, n = mapping->num_perms;
 		access_vector_t result;
 
-		for (i=0, result=0; i<current_mapping[tclass].num_perms; i++)
-			if (avd->allowed & current_mapping[tclass].perms[i])
+		for (i = 0, result = 0; i < n; i++) {
+			if (avd->allowed & mapping->perms[i])
+				result |= 1<<i;
+			else if (allow_unknown && !mapping->perms[i])
 				result |= 1<<i;
+		}
 		avd->allowed = result;
 
-		for (i=0, result=0; i<current_mapping[tclass].num_perms; i++)
-			if (avd->decided & current_mapping[tclass].perms[i])
+		for (i = 0, result = 0; i < n; i++) {
+			if (avd->decided & mapping->perms[i])
+				result |= 1<<i;
+			else if (allow_unknown && !mapping->perms[i])
 				result |= 1<<i;
+		}
 		avd->decided = result;
 
-		for (i=0, result=0; i<current_mapping[tclass].num_perms; i++)
-			if (avd->auditallow & current_mapping[tclass].perms[i])
+		for (i = 0, result = 0; i < n; i++)
+			if (avd->auditallow & mapping->perms[i])
 				result |= 1<<i;
 		avd->auditallow = result;
 
-		for (i=0, result=0; i<current_mapping[tclass].num_perms; i++)
-			if (avd->auditdeny & current_mapping[tclass].perms[i])
+		for (i = 0, result = 0; i < n; i++) {
+			if (avd->auditdeny & mapping->perms[i])
 				result |= 1<<i;
+			else if (!allow_unknown && !mapping->perms[i])
+				result |= 1<<i;
+		}
+
+		/*
+		 * Make sure we audit denials for any permission check
+		 * beyond the mapping->num_perms since this indicates
+		 * a bug in the object manager.
+		 */
+		for (; i < (sizeof(result)*8); i++)
+			result |= 1<<i;
 		avd->auditdeny = result;
 	}
 }
diff --git a/libselinux/src/reject_unknown.c b/libselinux/src/reject_unknown.c
new file mode 100644
index 00000000..5c1d3605
--- /dev/null
+++ b/libselinux/src/reject_unknown.c
@@ -0,0 +1,40 @@
+#include <unistd.h>
+#include <sys/types.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <string.h>
+#include "selinux_internal.h"
+#include "policy.h"
+#include <stdio.h>
+#include <limits.h>
+
+int security_reject_unknown(void)
+{
+	int fd, ret, reject_unknown = 0;
+	char path[PATH_MAX];
+	char buf[20];
+
+	if (!selinux_mnt) {
+		errno = ENOENT;
+		return -1;
+	}
+
+	snprintf(path, sizeof(path), "%s/reject_unknown", selinux_mnt);
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return -1;
+
+	memset(buf, 0, sizeof(buf));
+	ret = read(fd, buf, sizeof(buf) - 1);
+	close(fd);
+	if (ret < 0)
+		return -1;
+
+	if (sscanf(buf, "%d", &reject_unknown) != 1)
+		return -1;
+
+	return reject_unknown;
+}
+
+hidden_def(security_reject_unknown);
diff --git a/libselinux/src/selinux_internal.h b/libselinux/src/selinux_internal.h
index dfc421cc..70b5025d 100644
--- a/libselinux/src/selinux_internal.h
+++ b/libselinux/src/selinux_internal.h
@@ -59,6 +59,7 @@ hidden_proto(selinux_mkload_policy)
     hidden_proto(security_getenforce)
     hidden_proto(security_setenforce)
     hidden_proto(security_deny_unknown)
+    hidden_proto(security_reject_unknown)
     hidden_proto(security_get_checkreqprot)
     hidden_proto(selinux_boolean_sub)
     hidden_proto(selinux_current_policy_path)
-- 
2.20.1


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