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

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Sep 15, 2026 at 03:15:03PM +0800, Linkui Xiao wrote:
> From: Linkui Xiao <xiaolinkui@kylinos.cn>
> 
> 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.
> 
> Moreover, nh_saddr is only meaningful when nh_saddr_genid matches
> dev_addr_genid, as established by 195374d89368. fib_select_multipath()

It's established by commit 436c3b66ec98 ("ipv4: Invalidate nexthop cache
nh_saddr more correctly.")

> skips that validation, so it can score a nexthop using a stale source
> address and skew the ECMP selection.
> 
> Annotate both reads with READ_ONCE() and add the missing genid check,
> mirroring fib_result_prefsrc().
> 
> Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches source address")
> Signed-off-by: Linkui Xiao <xiaolinkui@kylinos.cn>
> ---
> V1: https://lore.kernel.org/all/CANn89iJhFL2LNQCXTANQAx8B1GVdGTJgrDwPTZOjbH8k1ygtsg@mail.gmail.com/
> 
> V2:
> - point Fixes: at 32607a332cfe, which introduced the bare reader
> - add the missing genid check as suggested by Eric Dumazet
> 
>  net/ipv4/fib_semantics.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 7a362f2e2c2b..e2bfc11aea09 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -2184,6 +2184,7 @@ 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);
> @@ -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;
>  		if (hash <= nh_upper_bound)
>  			nh_score++;

The cached generation ID can remain stale for a long time and during
this time we will silently downgrade the nexthop selection to be hash
based. We need to refresh it in case there is a mismatch. Something
like:

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index e2bfc11aea09..5c9021ea3a79 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -2176,6 +2176,15 @@ static bool fib_good_nh(const struct fib_nh *nh)
 	return !!(state & NUD_VALID);
 }
 
+static __be32 fib_nh_saddr(struct net *net, const struct fib_info *fi,
+			   struct fib_nh *nh, int genid)
+{
+	if (READ_ONCE(nh->nh_saddr_genid) == genid)
+		return READ_ONCE(nh->nh_saddr);
+
+	return fib_info_update_nhc_saddr(net, &nh->nh_common, fi->fib_scope);
+}
+
 void fib_select_multipath(struct fib_result *res, int hash,
 			  const struct flowi4 *fl4)
 {
@@ -2206,9 +2215,7 @@ void fib_select_multipath(struct fib_result *res, int hash,
 		    (use_neigh && !fib_good_nh(nexthop_nh)))
 			continue;
 
-		if (saddr &&
-		    READ_ONCE(nexthop_nh->nh_saddr_genid) == genid &&
-		    READ_ONCE(nexthop_nh->nh_saddr) == saddr)
+		if (saddr && fib_nh_saddr(net, fi, nexthop_nh, genid) == saddr)
 			nh_score += 2;
 		if (hash <= nh_upper_bound)
 			nh_score++;