lsm: Only build lsm_audit.c if CONFIG_SECURITY and CONFIG_AUDIT are set

When CONFIG_AUDIT is set, its CONFIG_NET dependency is also set, and the
dev_get_by_index and init_net symbols (used by dump_common_audit_data)
are found by the linker.  dump_common_audit_data() should then failed to
build when CONFIG_NET is not set. However, because the compiler is
smart, it knows that audit_log_start() always return NULL when
!CONFIG_AUDIT, and it doesn't build the body of common_lsm_audit().  As
a side effect, dump_common_audit_data() is not built and the linker
doesn't error out because of missing symbols.

Let's only build lsm_audit.o when CONFIG_SECURITY and CONFIG_AUDIT are
both set, which is checked with the new CONFIG_HAS_SECURITY_AUDIT.

ipv4_skb_to_auditdata() and ipv6_skb_to_auditdata() are only used by
Smack if CONFIG_AUDIT is set, so they don't need fake implementations.

Because common_lsm_audit() is used in multiple places without
CONFIG_AUDIT checks, add a fake implementation.

Link: https://lore.kernel.org/r/[email protected]
Cc: Casey Schaufler <[email protected]>
Cc: James Morris <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Serge E. Hallyn <[email protected]>
Signed-off-by: Mickaël Salaün <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

include/linux/lsm_audit.h

@@ -116,14 +116,28 @@ struct common_audit_data {
 #define v4info fam.v4
 #define v6info fam.v6
 
+#ifdef CONFIG_AUDIT
+
 int ipv4_skb_to_auditdata(struct sk_buff *skb,
 		struct common_audit_data *ad, u8 *proto);
 
+#if IS_ENABLED(CONFIG_IPV6)
 int ipv6_skb_to_auditdata(struct sk_buff *skb,
 		struct common_audit_data *ad, u8 *proto);
+#endif /* IS_ENABLED(CONFIG_IPV6) */
 
 void common_lsm_audit(struct common_audit_data *a,
 	void (*pre_audit)(struct audit_buffer *, void *),
 	void (*post_audit)(struct audit_buffer *, void *));
 
+#else /* CONFIG_AUDIT */
+
+static inline void common_lsm_audit(struct common_audit_data *a,
+	void (*pre_audit)(struct audit_buffer *, void *),
+	void (*post_audit)(struct audit_buffer *, void *))
+{
+}
+
+#endif /* CONFIG_AUDIT */
+
 #endif

security/Kconfig

@@ -64,6 +64,11 @@ config SECURITY
 
 	  If you are unsure how to answer this question, answer N.
 
+config HAS_SECURITY_AUDIT
+	def_bool y
+	depends on AUDIT
+	depends on SECURITY
+
 config SECURITYFS
 	bool "Enable the securityfs filesystem"
 	help

security/Makefile

@@ -15,7 +15,7 @@ obj-$(CONFIG_SECURITY)			+= security.o
 obj-$(CONFIG_SECURITYFS)		+= inode.o
 obj-$(CONFIG_SECURITY_SELINUX)		+= selinux/
 obj-$(CONFIG_SECURITY_SMACK)		+= smack/
-obj-$(CONFIG_SECURITY)			+= lsm_audit.o
+obj-$(CONFIG_HAS_SECURITY_AUDIT)	+= lsm_audit.o
 obj-$(CONFIG_SECURITY_TOMOYO)		+= tomoyo/
 obj-$(CONFIG_SECURITY_APPARMOR)		+= apparmor/
 obj-$(CONFIG_SECURITY_YAMA)		+= yama/