Re: [PATCH 6/6] ndp: Send unsolicited Router Advertisements

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 13, 2024 at 02:14:16AM +0100, Stefano Brivio wrote:
> Kind of nits only, the whole series looks good to me otherwise:
> 
> On Tue, 12 Nov 2024 15:06:18 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Currently, our NDP implementation only sends Router Advertisements (RA)
> > when it receives a Router Solicitation (RS) from the guest.  However,
> > RFC 4861 requires that we periodically send unsolicited RAs.
> > 
> > Linux as a guest also requires this: it will send an RS when a link first
> > comes up, but the route it gets from this will have a finite lifetime (we
> > set this to 65535s, the maximum allowed, around 18 hours).  When that
> > expires the guest will not send a new RS, but instead expects the route to
> > have been renewed (if still valid) by an unsolicited RA.
> > 
> > Implement sending unsolicited RAs on a partially randomised timer, as
> > required by RFC 4861.  The RFC also specifies that solicited RAs should
> > also be delayed, or even not omitted, if the next unsolicited RA is soon
> 
> s/not//

Fixed.

> > enough.  For now we don't do that, always sending an immediate RA in
> > response to an RS.  We can get away with this because in our use cases
> > we expect to just have passt itself and the guest on the link, rather than
> > a large broadcast domain.
> > 
> > Link: https://github.com/kubevirt/kubevirt/issues/13191
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  ip.h    |  9 +++++++++
> >  ndp.c   | 41 +++++++++++++++++++++++++++++++++++++++++
> >  ndp.h   |  3 +++
> >  passt.c |  3 +++
> >  4 files changed, 56 insertions(+)
> > 
> > diff --git a/ip.h b/ip.h
> > index b8d4a5b..0742612 100644
> > --- a/ip.h
> > +++ b/ip.h
> > @@ -92,4 +92,13 @@ struct ipv6_opt_hdr {
> >  
> >  char *ipv6_l4hdr(const struct pool *p, int idx, size_t offset, uint8_t *proto,
> >  		 size_t *dlen);
> > +
> > +/* IPv6 link-local all-nodes multicast adddress, ff02::1 */
> > +static const struct in6_addr in6addr_ll_all_nodes = {
> > +	.s6_addr = {
> > +		0xff, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
> > +		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
> > +	},
> > +};
> > +
> >  #endif /* IP_H */
> > diff --git a/ndp.c b/ndp.c
> > index c5fbccf..8c019df 100644
> > --- a/ndp.c
> > +++ b/ndp.c
> > @@ -372,3 +372,44 @@ int ndp(const struct ctx *c, const struct icmp6hdr *ih,
> >  
> >  	return 1;
> >  }
> > +
> > +/* Default interval between unsolicited RAs (seconds) */
> > +#define DEFAULT_MAX_RTR_ADV_INTERVAL	600	/* RFC 4861, 6.2.1 */
> > +
> > +/* Minimum required interval between RAs (seconds) */
> > +#define MIN_DELAY_BETWEEN_RAS		3	/* RFC 4861, 10 */
> 
> By the way, I think this is still all correct, as I quickly double
> checked it against RFC 8319. If you're not aware: see also sections 3
> and 4 there.

Ah, thanks for checking.  I didn't think to look for a superseding RFC.

> > +
> > +static time_t next_ra;
> > +
> > +/**
> > + * ndp_timer() - Send unsolicited NDP messages if necessary
> > + * @c:		Execution context
> > + * @now:	Current (monotonic) time
> > + */
> > +void ndp_timer(const struct ctx *c, const struct timespec *now)
> > +{
> > +	time_t max_rtr_adv_interval = DEFAULT_MAX_RTR_ADV_INTERVAL;
> > +	time_t min_rtr_adv_interval, interval;
> > +
> > +	if (c->no_ra || now->tv_sec < next_ra)
> > +		return;
> > +
> > +	/* We must advertise before the route's lifetime expires */
> > +	max_rtr_adv_interval = MIN(max_rtr_adv_interval, RT_LIFETIME - 1);
> > +
> > +	/* But we must not go smaller than the minimum delay */
> > +	max_rtr_adv_interval = MAX(max_rtr_adv_interval, MIN_DELAY_BETWEEN_RAS);
> > +
> > +	/* RFC 4861, 6.2.1 */
> > +	min_rtr_adv_interval = MAX(max_rtr_adv_interval / 3,
> > +				   MIN_DELAY_BETWEEN_RAS);
> > +
> > +	interval = min_rtr_adv_interval +
> > +		random() % (max_rtr_adv_interval - min_rtr_adv_interval);
> 
> So, I would have called srandom() before this, especially in the case
> we get, one day, two instances of passt advertising at the same
> time.

Good point, I'll add that.

> But Coverity is more annoying than I am and reports:
> 
> /home/sbrivio/passt/ndp.c:408:3:
>   Type: Calling risky function (DC.WEAK_CRYPTO)
> 
> /home/sbrivio/passt/ndp.c:408:3:
>   dont_call: "random" should not be used for security-related applications, because linear congruential algorithms are too easy to break.
> /home/sbrivio/passt/ndp.c:408:3:
>   remediation: Use a compliant random number generator, such as "/dev/random" or "/dev/urandom" on Unix-like systems, and CNG (Cryptography API: Next Generation) on Windows.
> 
> Of course it's all bogus but I would have a *slight* preference to get
> rid of this, by either picking a fixed interval deviation at the
> beginning with getrandom(), or using something like tcp_init_seq()
> modulo something << 600.

I don't think the latter approach really works.  Using the time as the
basic input means its still subject to two passt instances (say)
starting at the same time on the same link picking the same values and
keeping on sending their announcements in lockstep.

Picking a random interval once is better.  Still, I don't particularly
like deviating from the RFC's recommendations just to keep a fussy
tool happy.

> Alternatively, we could also keep /dev/random or /dev/urandom open but
> it looks totally overkill. At that point I'd rather keep random()
> here.

Also a waste of /dev/random entropy, IMO.

Surely there must be some way we can suppress the Coverity whinge.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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