netdev - Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy

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Message-ID: <20210208104143.60a6d730@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 8 Feb 2021 10:41:43 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Arjun Roy <arjunroy.kdev@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, arjunroy@...gle.com, edumazet@...gle.com,
        soheil@...gle.com, David Ahern <dsahern@...il.com>
Subject: Re: [net-next v2] tcp: Explicitly mark reserved field in
 tcp_zerocopy_receive args.

On Sun, 7 Feb 2021 10:26:54 +0200 Leon Romanovsky wrote:
> 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 <arjunroy@...gle.com>
> > >
> > > 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)".

Which check? There's a check which truncates (writes back to user space
len = min(len, sizeof(zc)). Application can still pass garbage beyond
sizeof(zc) and syscall may start failing in the future if sizeof(zc)
changes.