selinux: check sidtab limit before adding a new entry

We need to error out when trying to add an entry above SIDTAB_MAX in
sidtab_reverse_lookup() to avoid overflow on the odd chance that this
happens.

Cc: stable@vger.kernel.org
Fixes: ee1a84fd ("selinux: overhaul sidtab to fix bug and improve performance")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/sidtab.c

@@ -286,6 +286,11 @@ static int sidtab_reverse_lookup(struct sidtab *s, struct context *context,
 		++count;
 	}
 
+	/* bail out if we already reached max entries */
+	rc = -EOVERFLOW;
+	if (count >= SIDTAB_MAX)
+		goto out_unlock;
+
 	/* insert context into new entry */
 	rc = -ENOMEM;
 	dst = sidtab_do_lookup(s, count, 1);