Re: [PATCH] ipv4: fib: annotate data-race around nh->nh_saddr

[Eric Dumazet]

On Mon, Sep 14, 2026 at 12:57 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Mon, Sep 14, 2026 at 12:51 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> >
> > On Fri, Sep 11, 2026 at 12:38 AM Linkui Xiao <xiaolinkui@xxxxxxx> wrote:
> > >
> > > From: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
> > >
> > > fib_select_multipath() compares nexthop_nh->nh_saddr against the flow
> > > source address with no lock held, while fib_info_update_nhc_saddr()
> > > stores a new value from another CPU as soon as the preferred source
> > > address of the egress device changes.
> > >
> > > Commit 195374d89368 ("ipv4: fib: annotate races around nh->nh_saddr_genid
> > > and nh->nh_saddr") added WRITE_ONCE() on the store side and READ_ONCE()
> > > in fib_result_prefsrc() after syzbot reported
> > >
> > >         BUG: KCSAN: data-race in fib_select_path / fib_select_path
> > >
> > > but it only covered that reader. fib_select_multipath(), reached from
> > > fib_select_path(), is a second lockless reader of nh->nh_saddr and was
> > > left bare.
> >
> > 32607a332cfe added the reader after 195374d89368.
> >
>
> Indeed, please put in V2:
>
> Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches
> source address")

Adding Willem

It seems that this code also lacks a check against genid?

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 50e96f86ca59ab164f764f5d2185d158c14a4a6a..5a8bbcb65daaada1c3ae96339cabc2e801653dac
100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -2185,6 +2185,7 @@ void fib_select_multipath(struct fib_result
*res, int hash,
        bool use_neigh;
        int score = -1;
        __be32 saddr;
+       int genid;

        if (unlikely(res->fi->nh)) {
                nexthop_path_fib_result(res, hash);
@@ -2193,6 +2194,7 @@ 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;
@@ -2205,7 +2207,9 @@ 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;
                if (hash <= nh_upper_bound)
                        nh_score++;