audit: add record for multiple task security contexts

Replace the single skb pointer in an audit_buffer with a list of
skb pointers. Add the audit_stamp information to the audit_buffer as
there's no guarantee that there will be an audit_context containing
the stamp associated with the event. At audit_log_end() time create
auxiliary records as have been added to the list. Functions are
created to manage the skb list in the audit_buffer.

Create a new audit record AUDIT_MAC_TASK_CONTEXTS.
An example of the MAC_TASK_CONTEXTS record is:

type=MAC_TASK_CONTEXTS
msg=audit(1600880931.832:113)
subj_apparmor=unconfined
subj_smack=_

When an audit event includes a AUDIT_MAC_TASK_CONTEXTS record the
"subj=" field in other records in the event will be "subj=?".
An AUDIT_MAC_TASK_CONTEXTS record is supplied when the system has
multiple security modules that may make access decisions based on a
subject security context.

Refactor audit_log_task_context(), creating a new audit_log_subj_ctx().
This is used in netlabel auditing to provide multiple subject security
contexts as necessary.

Suggested-by: Paul Moore <[e-mail]>
Signed-off-by: Casey Schaufler <[e-mail]>
[PM: subj tweak, audit example readability indents]
Signed-off-by: Paul Moore <[e-mail]>

include/linux/audit.h

···
 struct audit_tree;
 struct sk_buff;
 struct kern_ipc_perm;
+struct lsm_id;
+struct lsm_prop;
 
 struct audit_krule {
 	u32			pflags;
···
 #define AUDIT_TTY_ENABLE	BIT(0)
 #define AUDIT_TTY_LOG_PASSWD	BIT(1)
 
+/* bit values for audit_cfg_lsm */
+#define AUDIT_CFG_LSM_SECCTX_SUBJECT	BIT(0)
+
 struct filename;
 
 #define AUDIT_OFF	0
···
 						  const char *operation);
 extern void		    audit_log_lost(const char *message);
 
+extern int audit_log_subj_ctx(struct audit_buffer *ab, struct lsm_prop *prop);
 extern int audit_log_task_context(struct audit_buffer *ab);
 extern void audit_log_task_info(struct audit_buffer *ab);
 
···
 extern u32 audit_enabled;
 
 extern int audit_signal_info(int sig, struct task_struct *t);
+
+extern void audit_cfg_lsm(const struct lsm_id *lsmid, int flags);
 
 #else /* CONFIG_AUDIT */
 static inline __printf(4, 5)
···
 { }
 static inline void audit_log_path_denied(int type, const char *operation)
 { }
+static inline int audit_log_subj_ctx(struct audit_buffer *ab,
+				     struct lsm_prop *prop)
+{
+	return 0;
+}
 static inline int audit_log_task_context(struct audit_buffer *ab)
 {
 	return 0;
···
 {
 	return 0;
 }
+
+static inline void audit_cfg_lsm(const struct lsm_id *lsmid, int flags)
+{ }
 
 #endif /* CONFIG_AUDIT */
 

include/uapi/linux/audit.h

···
 #define AUDIT_IPE_POLICY_LOAD	1422	/* IPE policy load */
 #define AUDIT_LANDLOCK_ACCESS	1423	/* Landlock denial */
 #define AUDIT_LANDLOCK_DOMAIN	1424	/* Landlock domain status */
+#define AUDIT_MAC_TASK_CONTEXTS	1425	/* Multiple LSM task contexts */
 
 #define AUDIT_FIRST_KERN_ANOM_MSG   1700
 #define AUDIT_LAST_KERN_ANOM_MSG    1799

kernel/audit.c

···
 #include <net/netlink.h>
 #include <linux/skbuff.h>
 #include <linux/security.h>
+#include <linux/lsm_hooks.h>
 #include <linux/freezer.h>
 #include <linux/pid_namespace.h>
 #include <net/netns/generic.h>
···
 
 /* private audit network namespace index */
 static unsigned int audit_net_id;
+
+/* Number of modules that provide a security context.
+   List of lsms that provide a security context */
+static u32 audit_subj_secctx_cnt;
+static const struct lsm_id *audit_subj_lsms[MAX_LSM_COUNT];
 
 /**
  * struct audit_net - audit private network namespace data
···
  * to place it on a transmit queue.  Multiple audit_buffers can be in
  * use simultaneously. */
 struct audit_buffer {
-	struct sk_buff       *skb;	/* formatted skb ready to send */
+	struct sk_buff       *skb;	/* the skb for audit_log functions */
+	struct sk_buff_head  skb_list;	/* formatted skbs, ready to send */
 	struct audit_context *ctx;	/* NULL or associated context */
+	struct audit_stamp   stamp;	/* audit stamp for these records */
 	gfp_t		     gfp_mask;
 };
 
···
 	rcu_read_unlock();
 
 	return pid;
+}
+
+/**
+ * audit_cfg_lsm - Identify a security module as providing a secctx.
+ * @lsmid: LSM identity
+ * @flags: which contexts are provided
+ *
+ * Description:
+ * Increments the count of the security modules providing a secctx.
+ * If the LSM id is already in the list leave it alone.
+ */
+void audit_cfg_lsm(const struct lsm_id *lsmid, int flags)
+{
+	int i;
+
+	if (flags & AUDIT_CFG_LSM_SECCTX_SUBJECT) {
+		for (i = 0 ; i < audit_subj_secctx_cnt; i++)
+			if (audit_subj_lsms[i] == lsmid)
+				return;
+		audit_subj_lsms[audit_subj_secctx_cnt++] = lsmid;
+	}
 }
 
 /**
···
 
 static void audit_buffer_free(struct audit_buffer *ab)
 {
+	struct sk_buff *skb;
+
 	if (!ab)
 		return;
 
-	kfree_skb(ab->skb);
+	while ((skb = skb_dequeue(&ab->skb_list)))
+		kfree_skb(skb);
 	kmem_cache_free(audit_buffer_cache, ab);
 }
 
···
 	ab->skb = nlmsg_new(AUDIT_BUFSIZ, gfp_mask);
 	if (!ab->skb)
 		goto err;
+
+	skb_queue_head_init(&ab->skb_list);
+	skb_queue_tail(&ab->skb_list, ab->skb);
+
 	if (!nlmsg_put(ab->skb, 0, 0, type, 0, 0))
 		goto err;
 
···
 				     int type)
 {
 	struct audit_buffer *ab;
-	struct audit_stamp stamp;
 
 	if (audit_initialized != AUDIT_INITIALIZED)
 		return NULL;
···
 		return NULL;
 	}
 
-	audit_get_stamp(ab->ctx, &stamp);
+	audit_get_stamp(ab->ctx, &ab->stamp);
 	/* cancel dummy context to enable supporting records */
 	if (ctx)
 		ctx->dummy = 0;
 	audit_log_format(ab, "audit(%llu.%03lu:%u): ",
-			 (unsigned long long)stamp.ctime.tv_sec,
-			 stamp.ctime.tv_nsec/1000000,
-			 stamp.serial);
+			 (unsigned long long)ab->stamp.ctime.tv_sec,
+			 ab->stamp.ctime.tv_nsec/1000000,
+			 ab->stamp.serial);
 
 	return ab;
 }
···
 		audit_log_format(ab, "(null)");
 }
 
-int audit_log_task_context(struct audit_buffer *ab)
+/**
+ * audit_buffer_aux_new - Add an aux record buffer to the skb list
+ * @ab: audit_buffer
+ * @type: message type
+ *
+ * Aux records are allocated and added to the skb list of
+ * the "main" record. The ab->skb is reset to point to the
+ * aux record on its creation. When the aux record in complete
+ * ab->skb has to be reset to point to the "main" record.
+ * This allows the audit_log_ functions to be ignorant of
+ * which kind of record it is logging to. It also avoids adding
+ * special data for aux records.
+ *
+ * On success ab->skb will point to the new aux record.
+ * Returns 0 on success, -ENOMEM should allocation fail.
+ */
+static int audit_buffer_aux_new(struct audit_buffer *ab, int type)
 {
-	struct lsm_prop prop;
-	struct lsm_context ctx;
-	int error;
+	WARN_ON(ab->skb != skb_peek(&ab->skb_list));
 
-	security_current_getlsmprop_subj(&prop);
-	if (!lsmprop_is_set(&prop))
+	ab->skb = nlmsg_new(AUDIT_BUFSIZ, ab->gfp_mask);
+	if (!ab->skb)
+		goto err;
+	if (!nlmsg_put(ab->skb, 0, 0, type, 0, 0))
+		goto err;
+	skb_queue_tail(&ab->skb_list, ab->skb);
+
+	audit_log_format(ab, "audit(%llu.%03lu:%u): ",
+			 (unsigned long long)ab->stamp.ctime.tv_sec,
+			 ab->stamp.ctime.tv_nsec/1000000,
+			 ab->stamp.serial);
+
+	return 0;
+
+err:
+	kfree_skb(ab->skb);
+	ab->skb = skb_peek(&ab->skb_list);
+	return -ENOMEM;
+}
+
+/**
+ * audit_buffer_aux_end - Switch back to the "main" record from an aux record
+ * @ab: audit_buffer
+ *
+ * Restores the "main" audit record to ab->skb.
+ */
+static void audit_buffer_aux_end(struct audit_buffer *ab)
+{
+	ab->skb = skb_peek(&ab->skb_list);
+}
+
+/**
+ * audit_log_subj_ctx - Add LSM subject information
+ * @ab: audit_buffer
+ * @prop: LSM subject properties.
+ *
+ * Add a subj= field and, if necessary, a AUDIT_MAC_TASK_CONTEXTS record.
+ */
+int audit_log_subj_ctx(struct audit_buffer *ab, struct lsm_prop *prop)
+{
+	struct lsm_context ctx;
+	char *space = "";
+	int error;
+	int i;
+
+	security_current_getlsmprop_subj(prop);
+	if (!lsmprop_is_set(prop))
 		return 0;
 
-	error = security_lsmprop_to_secctx(&prop, &ctx, LSM_ID_UNDEF);
-	if (error < 0) {
-		if (error != -EINVAL)
-			goto error_path;
+	if (audit_subj_secctx_cnt < 2) {
+		error = security_lsmprop_to_secctx(prop, &ctx, LSM_ID_UNDEF);
+		if (error < 0) {
+			if (error != -EINVAL)
+				goto error_path;
+			return 0;
+		}
+		audit_log_format(ab, " subj=%s", ctx.context);
+		security_release_secctx(&ctx);
 		return 0;
 	}
+	/* Multiple LSMs provide contexts. Include an aux record. */
+	audit_log_format(ab, " subj=?");
+	error = audit_buffer_aux_new(ab, AUDIT_MAC_TASK_CONTEXTS);
+	if (error)
+		goto error_path;
 
-	audit_log_format(ab, " subj=%s", ctx.context);
-	security_release_secctx(&ctx);
+	for (i = 0; i < audit_subj_secctx_cnt; i++) {
+		error = security_lsmprop_to_secctx(prop, &ctx,
+						   audit_subj_lsms[i]->id);
+		if (error < 0) {
+			/*
+			 * Don't print anything. An LSM like BPF could
+			 * claim to support contexts, but only do so under
+			 * certain conditions.
+			 */
+			if (error == -EOPNOTSUPP)
+				continue;
+			if (error != -EINVAL)
+				audit_panic("error in audit_log_subj_ctx");
+		} else {
+			audit_log_format(ab, "%ssubj_%s=%s", space,
+					 audit_subj_lsms[i]->name, ctx.context);
+			space = " ";
+			security_release_secctx(&ctx);
+		}
+	}
+	audit_buffer_aux_end(ab);
 	return 0;
 
 error_path:
-	audit_panic("error in audit_log_task_context");
+	audit_panic("error in audit_log_subj_ctx");
 	return error;
+}
+EXPORT_SYMBOL(audit_log_subj_ctx);
+
+int audit_log_task_context(struct audit_buffer *ab)
+{
+	struct lsm_prop prop;
+
+	security_current_getlsmprop_subj(&prop);
+	return audit_log_subj_ctx(ab, &prop);
 }
 EXPORT_SYMBOL(audit_log_task_context);
 
···
 }
 
 /**
+ * __audit_log_end - enqueue one audit record
+ * @skb: the buffer to send
+ */
+static void __audit_log_end(struct sk_buff *skb)
+{
+	struct nlmsghdr *nlh;
+
+	if (audit_rate_check()) {
+		/* setup the netlink header, see the comments in
+		 * kauditd_send_multicast_skb() for length quirks */
+		nlh = nlmsg_hdr(skb);
+		nlh->nlmsg_len = skb->len - NLMSG_HDRLEN;
+
+		/* queue the netlink packet */
+		skb_queue_tail(&audit_queue, skb);
+	} else
+		audit_log_lost("rate limit exceeded");
+}
+
+/**
  * audit_log_end - end one audit record
  * @ab: the audit_buffer
  *
···
 void audit_log_end(struct audit_buffer *ab)
 {
 	struct sk_buff *skb;
-	struct nlmsghdr *nlh;
 
 	if (!ab)
 		return;
 
-	if (audit_rate_check()) {
-		skb = ab->skb;
-		ab->skb = NULL;
+	while ((skb = skb_dequeue(&ab->skb_list)))
+		__audit_log_end(skb);
 
-		/* setup the netlink header, see the comments in
-		 * kauditd_send_multicast_skb() for length quirks */
-		nlh = nlmsg_hdr(skb);
-		nlh->nlmsg_len = skb->len - NLMSG_HDRLEN;
-
-		/* queue the netlink packet and poke the kauditd thread */
-		skb_queue_tail(&audit_queue, skb);
-		wake_up_interruptible(&kauditd_wait);
-	} else
-		audit_log_lost("rate limit exceeded");
+	/* poke the kauditd thread */
+	wake_up_interruptible(&kauditd_wait);
 
 	audit_buffer_free(ab);
 }

net/netlabel/netlabel_user.c

···
 					       struct netlbl_audit *audit_info)
 {
 	struct audit_buffer *audit_buf;
-	struct lsm_context ctx;
 
 	if (audit_enabled == AUDIT_OFF)
 		return NULL;
···
 	audit_log_format(audit_buf, "netlabel: auid=%u ses=%u",
 			 from_kuid(&init_user_ns, audit_info->loginuid),
 			 audit_info->sessionid);
-
-	if (lsmprop_is_set(&audit_info->prop) &&
-	    security_lsmprop_to_secctx(&audit_info->prop, &ctx,
-				       LSM_ID_UNDEF) > 0) {
-		audit_log_format(audit_buf, " subj=%s", ctx.context);
-		security_release_secctx(&ctx);
-	}
+	audit_log_subj_ctx(audit_buf, &audit_info->prop);
 
 	return audit_buf;
 }

security/apparmor/lsm.c

···
 	security_add_hooks(apparmor_hooks, ARRAY_SIZE(apparmor_hooks),
 				&apparmor_lsmid);
 
+	/* Inform the audit system that secctx is used */
+	audit_cfg_lsm(&apparmor_lsmid, AUDIT_CFG_LSM_SECCTX_SUBJECT);
+
 	/* Report that AppArmor successfully initialized */
 	apparmor_initialized = 1;
 	if (aa_g_profile_mode == APPARMOR_COMPLAIN)

security/selinux/hooks.c

···
 	/* Set the security state for the initial task. */
 	cred_init_security();
 
+	/* Inform the audit system that secctx is used */
+	audit_cfg_lsm(&selinux_lsmid, AUDIT_CFG_LSM_SECCTX_SUBJECT);
+
 	default_noexec = !(VM_DATA_DEFAULT_FLAGS & VM_EXEC);
 	if (!default_noexec)
 		pr_notice("SELinux:  virtual memory is executable by default\n");

security/smack/smack_lsm.c

···
 	/* initialize the smack_known_list */
 	init_smack_known_list();
 
+	/* Inform the audit system that secctx is used */
+	audit_cfg_lsm(&smack_lsmid, AUDIT_CFG_LSM_SECCTX_SUBJECT);
+
 	return 0;
 }