netdev - Re: [PATCH v2 bpf-next 1/5] bpf: Hooks for sys

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Date:   Thu, 24 May 2018 21:56:02 -0700
From:   Andrey Ignatov <rdna@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>, <davem@...emloft.net>, <kafai@...com>,
        <ast@...nel.org>, <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/5] bpf: Hooks for sys_sendmsg

Daniel Borkmann <daniel@...earbox.net> [Thu, 2018-05-24 18:00 -0700]:
> On 05/23/2018 01:40 AM, Andrey Ignatov wrote:
> [...]
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index ff4d4ba..a1f9ba2 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -900,6 +900,7 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> >  {
> >  	struct inet_sock *inet = inet_sk(sk);
> >  	struct udp_sock *up = udp_sk(sk);
> > +	DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
> >  	struct flowi4 fl4_stack;
> >  	struct flowi4 *fl4;
> >  	int ulen = len;
> > @@ -954,8 +955,7 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> >  	/*
> >  	 *	Get and verify the address.
> >  	 */
> > -	if (msg->msg_name) {
> > -		DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
> > +	if (usin) {
> >  		if (msg->msg_namelen < sizeof(*usin))
> >  			return -EINVAL;
> >  		if (usin->sin_family != AF_INET) {
> > @@ -1009,6 +1009,22 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> >  		rcu_read_unlock();
> >  	}
> >  
> > +	if (!connected) {
> > +		err = BPF_CGROUP_RUN_PROG_UDP4_SENDMSG_LOCK(sk,
> > +					    (struct sockaddr *)usin, &ipc.addr);
> > +		if (err)
> > +			goto out_free;
> > +		if (usin) {
> > +			if (usin->sin_port == 0) {
> > +				/* BPF program set invalid port. Reject it. */
> > +				err = -EINVAL;
> > +				goto out_free;
> > +			}
> > +			daddr = usin->sin_addr.s_addr;
> > +			dport = usin->sin_port;
> > +		}
> > +	}
> > +
> >  	saddr = ipc.addr;
> >  	ipc.addr = faddr = daddr;
> >  
> > diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> > index 2839c1b..67c44b5 100644
> > --- a/net/ipv6/udp.c
> > +++ b/net/ipv6/udp.c
> > @@ -1315,6 +1315,29 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> >  		fl6.saddr = np->saddr;
> >  	fl6.fl6_sport = inet->inet_sport;
> >  
> > +	if (!connected) {
> > +		err = BPF_CGROUP_RUN_PROG_UDP6_SENDMSG_LOCK(sk,
> > +					   (struct sockaddr *)sin6, &fl6.saddr);
> > +		if (err)
> > +			goto out_no_dst;
> > +		if (sin6) {
> > +			if (ipv6_addr_v4mapped(&sin6->sin6_addr)) {
> > +				/* BPF program rewrote IPv6-only by IPv4-mapped
> > +				 * IPv6. It's currently unsupported.
> > +				 */
> > +				err = -ENOTSUPP;
> > +				goto out_no_dst;
> > +			}
> > +			if (sin6->sin6_port == 0) {
> > +				/* BPF program set invalid port. Reject it. */
> > +				err = -EINVAL;
> > +				goto out_no_dst;
> > +			}
> > +			fl6.fl6_dport = sin6->sin6_port;
> > +			fl6.daddr = sin6->sin6_addr;
> > +		}
> 
> Hmm, this extra work here and in v4 case should probably all be done under
> the static key? Otherwise we'll do the extra work for checking sin6 and
> setting up fl6 twice?

Hm .. true, we can put the whole this block under static key (the main
one, since there are no others, but we can follow-up separately):

	if (cgroup_bpf_enabled && !connected) {

I'll send v3 with this change for both ipv6 and ipv4. Thanks.

As for the logic inside the `if`, I'll describe it just in case, since
some things may not be obvious.

There are two cases earlier in this function that can lead to
`connected = false`, either user specifies destination address (the 1st
`if (sin6)`) or/and user specifies ancillary data
(`if (msg->msg_controllen)`). 

Ancillary data can contain option to set source IP. So to simplify: if
user specifies source or destination we're in unconnected mode. 

Now imagine that we have connected socket and then user calls sendmsg
without setting destination (sin6 = NULL), but sets the source IP in
ancillary data at the same time. It will cause `connected = false` and
BPF prog will be run (it can e.g. override that source IP set by user),
but we have no sin6, that's why this `if (sin6)` is second time here.

On the other hand if sin6 is passed by user, it'll cause unconnected
mode as well and BPF prog has a chance to override IP and port in sin6
and in this case we have to update fl6 after BPF prog finishes. That's
why `fl6.daddr = sin6->sin6_addr;` the second time.

But I agree that work should be avoided when cgroup-bpf is disabled.

> Also, when not enabled, couldn't we run into the case
> of ipv6_addr_v4mapped() as well? If I'm spotting this right, then we would
> bail out though we shouldn't normally?

IPv4-mapped IPv6 case is handled earlier in this function and if user
passed IPv4-mapped IPv6, we don't get this far and call IPv4
udp_sendmsg() much earlier.

Same is true for port.

That's why this code wouldn't affect the logic for IPv4-mapped IPv6, but
again, you're right that we shouldn't do this extra work when cgroup-bpf
is disabled and I'll fix it.

> 
> > +	}
> > +
> >  	final_p = fl6_update_dst(&fl6, opt, &final);
> >  	if (final_p)
> >  		connected = false;
> > @@ -1394,6 +1417,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> >  
> >  out:
> >  	dst_release(dst);
> > +out_no_dst:
> >  	fl6_sock_release(flowlabel);
> >  	txopt_put(opt_to_free);
> >  	if (!err)
> > 
> 

-- 
Andrey Ignatov