[RFC PATCH] selinux: log invalid contexts in AVCs

[Ondrej Mosnacek <omosnace@redhat.com>]

In case a file has an invalid context set, in an AVC record generated
upon access to such file, the target context is always reported as
unlabeled. This patch adds new optional fields to the AVC record (slcon
and tlcon) that report the actual context string if it differs from the
one reported in scontext/tcontext. This is useful for diagnosing SELinux
denials.

To trigger an AVC that illustrates this situation:

    # setenforce 0
    # touch /tmp/testfile
    # setfattr -n security.selinux -v system_u:object_r:banana_t:s0 /tmp/testfile
    # runcon system_u:system_r:sshd_t:s0 cat /tmp/testfile

AVC before:

type=AVC msg=audit(1547801083.248:11): avc:  denied  { open } for  pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 tclass=file permissive=1

AVC after:

type=AVC msg=audit(1547801083.248:11): avc:  denied  { open } for  pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 tlcon=system_u:object_r:banana_t:s0 tclass=file permissive=1

Cc: Daniel Walsh <dwalsh@redhat.com>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1135683
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---
 security/selinux/avc.c | 49 +++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 20 deletions(-)

I'm not entirely sure about the record format here, so I'm Cc'ing
linux-audit and Steve for feedback. I went for optional fields to
minimize the size of the record, but maybe a different format is
preferred. If so, let me know and I'll do a respin.

Also, I accept suggestions for better field names than "slcon"/"tlcon"
("lcon" is meant as an acronym for "literal context", but I'm not sure
if that's a good name...).

diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index 9b63d8ee1687..4a181ed56e37 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -165,6 +165,32 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
 	audit_log_format(ab, " }");
 }
 
+static void avc_dump_sid(struct audit_buffer *ab, struct selinux_state *state,
+			 u32 sid, char type)
+{
+	int rc;
+	char *context, *lcontext;
+	u32 context_len, lcontext_len;
+
+	rc = security_sid_to_context(state, sid, &context, &context_len);
+	if (rc) {
+		audit_log_format(ab, "%csid=%d ", type, sid);
+		return;
+	}
+
+	audit_log_format(ab, "%ccontext=%s ", type, context);
+
+	/* in case of invalid context report also the actual context string */
+	rc = security_sid_to_context_force(state, sid, &lcontext,
+					   &lcontext_len);
+	if (!rc) {
+		if (strcmp(context, lcontext))
+			audit_log_format(ab, "%clcon=%s ", type, lcontext);
+		kfree(lcontext);
+	}
+	kfree(context);
+}
+
 /**
  * avc_dump_query - Display a SID pair and a class in human-readable form.
  * @ssid: source security identifier
@@ -174,28 +200,11 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
 static void avc_dump_query(struct audit_buffer *ab, struct selinux_state *state,
 			   u32 ssid, u32 tsid, u16 tclass)
 {
-	int rc;
-	char *scontext;
-	u32 scontext_len;
-
-	rc = security_sid_to_context(state, ssid, &scontext, &scontext_len);
-	if (rc)
-		audit_log_format(ab, "ssid=%d", ssid);
-	else {
-		audit_log_format(ab, "scontext=%s", scontext);
-		kfree(scontext);
-	}
-
-	rc = security_sid_to_context(state, tsid, &scontext, &scontext_len);
-	if (rc)
-		audit_log_format(ab, " tsid=%d", tsid);
-	else {
-		audit_log_format(ab, " tcontext=%s", scontext);
-		kfree(scontext);
-	}
+	avc_dump_sid(ab, state, ssid, 's');
+	avc_dump_sid(ab, state, tsid, 't');
 
 	BUG_ON(!tclass || tclass >= ARRAY_SIZE(secclass_map));
-	audit_log_format(ab, " tclass=%s", secclass_map[tclass-1].name);
+	audit_log_format(ab, "tclass=%s", secclass_map[tclass-1].name);
 }
 
 /**
-- 
2.20.1