Re: [PATCH net v2] ipv4: fib: fix data-race and stale genid check around nh->nh_saddr

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found 1 potential
issue(s) to consider:

Critical: 0 · High: 1 · Medium: 0 · Low: 0

- [High] In fib_select_multipath() (net/ipv4/fib_semantics.c), the newly
  added `READ_ONCE(nexthop_nh->nh_saddr_genid) == genid` test treats a…

--

ipv4: fib: fix data-race and stale genid check around nh->nh_saddr

This adds READ_ONCE() to the two lockless reads of nh->nh_saddr and
nh->nh_saddr_genid in fib_select_multipath(), and additionally requires the
cached nh_saddr_genid to equal a snapshot of net->ipv4.dev_addr_genid before
the source-address match can add its scoring bonus.  The stated intent is to
mirror fib_result_prefsrc().

> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 7a362f2e2c2bd..e2bfc11aea09c 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -2192,6 +2193,7 @@ void fib_select_multipath(struct fib_result *res, int hash,
>  
>  	use_neigh = READ_ONCE(net->ipv4.sysctl_fib_multipath_use_neigh);
>  	saddr = fl4 ? fl4->saddr : 0;
> +	genid = saddr ? atomic_read(&net->ipv4.dev_addr_genid) : 0;
>  
>  	change_nexthops(fi) {
>  		int nh_upper_bound, nh_score = 0;
> @@ -2204,7 +2206,9 @@ void fib_select_multipath(struct fib_result *res, int hash,
>  		    (use_neigh && !fib_good_nh(nexthop_nh)))
>  			continue;
>  
> -		if (saddr && nexthop_nh->nh_saddr == saddr)
> +		if (saddr &&
> +		    READ_ONCE(nexthop_nh->nh_saddr_genid) == genid &&
> +		    READ_ONCE(nexthop_nh->nh_saddr) == saddr)
>  			nh_score += 2;

[Severity: High]
Does this genid check turn a cache miss into a permanent disqualification,
so that the source-address preference from 32607a332cfe stops working after
any address event in the netns?

The function the commit message says is being mirrored treats a genid
mismatch as a miss and repopulates the cache:

net/ipv4/fib_semantics.c:fib_result_prefsrc() {
	...
		if (READ_ONCE(nh->nh_saddr_genid) ==
		    atomic_read(&net->ipv4.dev_addr_genid))
			return READ_ONCE(nh->nh_saddr);
	}

	return fib_info_update_nhc_saddr(net, nhc, res->fi->fib_scope);
}

fib_select_multipath() has no such fallback: on mismatch the nexthop simply
loses nh_score += 2, and nothing recomputes nh_saddr for it.

nh_saddr_genid is only stamped in fib_create_info() (once per fib_info at
route creation, for all nexthops), at nexthop object creation in
net/ipv4/nexthop.c, and by fib_result_prefsrc() for res->nhc alone.
Meanwhile dev_addr_genid is a per-netns counter bumped for any IPv4 address
event on any device:

net/ipv4/fib_frontend.c:fib_inetaddr_event() {
	...
		atomic_inc(&net->ipv4.dev_addr_genid);
		rt_cache_flush(net);

The same pattern appears for NETDEV_DOWN and in fib_netdev_event().  So after
one unrelated ip addr add/del, DHCP renew or NETDEV_UP, every fib_nh in the
netns has a stale nh_saddr_genid, and the accompanying rt_cache_flush()
forces established sockets into fresh lookups that already carry
fl4->saddr != 0.

Is there anything on that path that would refresh the cache?  The only lazy
refresher reachable from here is gated on the opposite condition:

net/ipv4/fib_semantics.c:fib_select_path() {
	...
check_saddr:
	if (!fl4->saddr) {
		...
			fl4->saddr = fib_result_prefsrc(net, res);

That block is skipped exactly when saddr is non-zero, which is the only case
the new check governs, and even when it does run it refreshes res->nhc only,
not the sibling nexthops walked by the scoring loop.  The other caller,
ip_mkroute_input() in net/ipv4/route.c, passes fl4 == NULL, so saddr is 0
there and the check has no effect on forwarding.

With that in place, for every output lookup that already has a source
address (socket bound before connect(), UDP or raw sendmsg with a fixed
source, and the post-flush re-lookups), does the +2 bonus become
unreachable, making nh_score == 3 impossible and reducing selection to plain
hash ECMP?  That would let a flow sourced from veth0's address egress via
veth1, which is the behaviour 32607a332cfe was written to avoid.

Would calling fib_info_update_nhc_saddr() on mismatch, or restricting this
patch to the READ_ONCE() annotations that the KCSAN report actually
justifies, be preferable here?  As it stands the Fixes: tag would carry this
behavioural change into stable.

[Severity: Low]
On the commit message wording:

    fib_info_update_nhc_saddr() stores a new value from another CPU as soon
    as the preferred source address of the egress device changes.

Is that accurate?  fib_info_update_nhc_saddr() is only called on demand
(from fib_result_prefsrc() on a genid mismatch, and at route creation), not
from the address-change notifier itself, which only bumps dev_addr_genid.

-- 
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