[PATCH v35 26/29] Audit: Add record for multiple task security contexts

[Casey Schaufler <casey@schaufler-ca.com>]

Create a new audit record AUDIT_MAC_TASK_CONTEXTS.
An example of the MAC_TASK_CONTEXTS (1420) record is:

    type=MAC_TASK_CONTEXTS[1420]
    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.

Functions are created to manage the skb list in the audit_buffer.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
---
 include/uapi/linux/audit.h |  1 +
 kernel/audit.c             | 93 +++++++++++++++++++++++++++++++++++---
 2 files changed, 88 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index 8eda133ca4c1..af0aaccfaf57 100644
--- a/include/uapi/linux/audit.h
+++ b/include/uapi/linux/audit.h
@@ -143,6 +143,7 @@
 #define AUDIT_MAC_UNLBL_STCDEL	1417	/* NetLabel: del a static label */
 #define AUDIT_MAC_CALIPSO_ADD	1418	/* NetLabel: add CALIPSO DOI entry */
 #define AUDIT_MAC_CALIPSO_DEL	1419	/* NetLabel: del CALIPSO DOI entry */
+#define AUDIT_MAC_TASK_CONTEXTS	1420	/* Multiple LSM task contexts */
 
 #define AUDIT_FIRST_KERN_ANOM_MSG   1700
 #define AUDIT_LAST_KERN_ANOM_MSG    1799
diff --git a/kernel/audit.c b/kernel/audit.c
index 4d44c05053b0..8ed2d717c217 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -2175,8 +2175,61 @@ void audit_log_key(struct audit_buffer *ab, char *key)
 		audit_log_format(ab, "(null)");
 }
 
+/**
+ * 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)
+{
+	WARN_ON(ab->skb != skb_peek(&ab->skb_list));
+
+	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);
+}
+
+
 int audit_log_task_context(struct audit_buffer *ab)
 {
+	int i;
 	int error;
 	struct lsmblob blob;
 	struct lsmcontext context;
@@ -2185,16 +2238,44 @@ int audit_log_task_context(struct audit_buffer *ab)
 	if (!lsmblob_is_set(&blob))
 		return 0;
 
-	error = security_secid_to_secctx(&blob, &context, LSMBLOB_FIRST);
+	if (!lsm_multiple_contexts()) {
+		error = security_secid_to_secctx(&blob, &context,
+						 LSMBLOB_FIRST);
+		if (error) {
+			if (error != -EINVAL)
+				goto error_path;
+			return 0;
+		}
 
-	if (error) {
-		if (error != -EINVAL)
+		audit_log_format(ab, " subj=%s", context.context);
+		security_release_secctx(&context);
+	} else {
+		/* 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;
-		return 0;
+		for (i = 0; i < LSMBLOB_ENTRIES; i++) {
+			if (blob.secid[i] == 0)
+				continue;
+			error = security_secid_to_secctx(&blob, &context, i);
+			if (error) {
+				audit_log_format(ab, "%ssubj_%s=?",
+						 i ? " " : "",
+						 lsm_slot_to_name(i));
+				if (error != -EINVAL)
+					audit_panic("error in audit_log_task_context");
+			} else {
+				audit_log_format(ab, "%ssubj_%s=%s",
+						 i ? " " : "",
+						 lsm_slot_to_name(i),
+						 context.context);
+				security_release_secctx(&context);
+			}
+		}
+		audit_buffer_aux_end(ab);
 	}
 
-	audit_log_format(ab, " subj=%s", context.context);
-	security_release_secctx(&context);
 	return 0;
 
 error_path:
-- 
2.35.1