Subject: Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy_receive args.

From: Leon Romanovsky <[email-masked]>
Date: Sun, 7 Feb 2021 10:26:54 +0200

On Sat, Feb 06, 2021 at 03:28:28PM -0800, Jakub Kicinski wrote:
> On Sat,  6 Feb 2021 12:36:48 -0800 Arjun Roy wrote:
> > From: Arjun Roy <[email-masked]>
> >
> > Explicitly define reserved field and require it to be 0-valued.
>
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index e1a17c6b473c..c8469c579ed8 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -4159,6 +4159,8 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
> >  		}
> >  		if (copy_from_user(&zc, optval, len))
> >  			return -EFAULT;
> > +		if (zc.reserved)
> > +			return -EINVAL;
> >  		lock_sock(sk);
> >  		err = tcp_zerocopy_receive(sk, &zc, &tss);
> >  		release_sock(sk);
>
> I was expecting we'd also throw in a check_zeroed_user().
> Either we can check if the buffer is zeroed all the way,
> or we can't and we shouldn't validate reserved either
>
> 	check_zeroed_user(optval + offsetof(reserved),
> 			  len - offsetof(reserved))
> ?

There is a check that len is not larger than zs and users can't give
large buffer.

I would say that is pretty safe to write "if (zc.reserved)".

Thanks