selinux: fix lsm_get_self_attr()

selinux_getselfattr() doesn't properly initialize the string pointer
it passes to selinux_lsm_getattr() which can cause a problem when an
attribute hasn't been explicitly set; selinux_lsm_getattr() returns
0/success, but does not set or initialize the string label/attribute.
Failure to properly initialize the string causes problems later in
selinux_getselfattr() when the function attempts to kfree() the
string.

Cc: Casey Schaufler <casey@schaufler-ca.com>
Fixes: 762c934 ("SELinux: Add selfattr hooks")
Suggested-by: Paul Moore <paul@paul-moore.com>
[PM: description changes as discussed in the thread]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -6559,7 +6559,7 @@ static int selinux_getselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
 			       size_t *size, u32 flags)
 {
 	int rc;
-	char *val;
+	char *val = NULL;
 	int val_len;
 
 	val_len = selinux_lsm_getattr(attr, current, &val);