[PATCH v2] lsm_audit,selinux: return early for possible NULL audit buffers

[Austin Kim <austindh.kim@gmail.com>]

From: Austin Kim <austin.kim@lge.com>

audit_log_start() may return NULL in below cases:

  - when audit is not initialized.
  - when audit backlog limit exceeds.

After the call to audit_log_start() is made and then possible NULL audit
buffer argument is passed to audit_log_*() functions, 
audit_log_*() functions return immediately in case of a NULL audit buffer 
argument.

But it is optimal to return early when audit_log_start() returns NULL,
because it is not necessary for audit_log_*() functions to be called with
NULL audit buffer argument.

So add exception handling for possible NULL audit buffers where
return value can be handled from callers.

Signed-off-by: Austin Kim <austin.kim@lge.com>
---
 security/selinux/hooks.c       | 4 ++++
 security/selinux/ss/services.c | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index b0032c42333e..9e84e6635f2f 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3325,6 +3325,8 @@ static int selinux_inode_setxattr(struct user_namespace *mnt_userns,
 			}
 			ab = audit_log_start(audit_context(),
 					     GFP_ATOMIC, AUDIT_SELINUX_ERR);
+			if (!ab)
+				return rc;
 			audit_log_format(ab, "op=setxattr invalid_context=");
 			audit_log_n_untrustedstring(ab, value, audit_size);
 			audit_log_end(ab);
@@ -6552,6 +6554,8 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
 				ab = audit_log_start(audit_context(),
 						     GFP_ATOMIC,
 						     AUDIT_SELINUX_ERR);
+				if (!ab)
+					return error;
 				audit_log_format(ab, "op=fscreate invalid_context=");
 				audit_log_n_untrustedstring(ab, value, audit_size);
 				audit_log_end(ab);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index d84c77f370dc..e5f1b2757a83 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -1673,6 +1673,8 @@ static int compute_sid_handle_invalid_context(
 	if (context_struct_to_string(policydb, newcontext, &n, &nlen))
 		goto out;
 	ab = audit_log_start(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR);
+	if (!ab)
+		goto out;
 	audit_log_format(ab,
 			 "op=security_compute_sid invalid_context=");
 	/* no need to record the NUL with untrusted strings */
-- 
2.20.1