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

[Eric Dumazet]

On Mon, Sep 14, 2026 at 7:37 PM Linkui Xiao <xiaolinkui@xxxxxxx> wrote:
>
>
>
> On 2026/9/15 04:21, Eric Dumazet wrote:
> > 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?
> Thanks Eric and Kuniyuki for the correction. I'll send a V2 with Fixes:
> 32607a332cfe.
>
> Kuniyuki, good catch on the missing genid check. nh_saddr is only
> meaningful when nh_saddr_genid matches dev_addr_genid, and
> fib_select_multipath() currently skips that validation. I'll fold the
> genid check into V2 along with the READ_ONCE annotations, unless you'd
> prefer to send it as a separate patch. Let me know.
>

I (Eric) was the one who mentioned the genid thing :)

Send a V2 with both bugs fixed.
Thanks

> Best regards,
> Linkui Xiao
> >
> > 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++;
>