bpf, cgroup: Fix problematic bounds check

Since ctx.optlen is signed, a larger value than max_value could be
passed, as it is later on used as unsigned, which causes a WARN_ON_ONCE
in the copy_to_user.

Fixes: 0d01da6 ("bpf: implement getsockopt and setsockopt hooks")
Signed-off-by: Loris Reiff <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Stanislav Fomichev <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Liblor

kernel/bpf/cgroup.c

@@ -1464,7 +1464,7 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
 		goto out;
 	}
 
-	if (ctx.optlen > max_optlen) {
+	if (ctx.optlen > max_optlen || ctx.optlen < 0) {
 		ret = -EFAULT;
 		goto out;
 	}