[PATCH net v3] ipv4: fib: fix data-race and stale genid check around nh->nh_saddr

[Linkui Xiao <xiaolinkui@126.com>]

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 commit 436c3b66ec98 ("ipv4: Invalidate
nexthop cache nh_saddr more correctly."). fib_select_multipath()
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 refresh the cached source
address via fib_info_update_nhc_saddr() when the genid does not match,
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

V3:
- correct the commit reference to 436c3b66ec98, which established the
  rule that nh_saddr is only valid when nh_saddr_genid matches
  dev_addr_genid (195374d89368 only added the READ_ONCE()/WRITE_ONCE()
  annotations)
- refresh the cached source address via fib_info_update_nhc_saddr()
  when the genid does not match, instead of skipping the comparison,
  to avoid silently downgrading ECMP selection to hash-based for a
  long stale period (suggested by Ido Schimmel)

 net/ipv4/fib_semantics.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 7a362f2e2c2b..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)
 {
@@ -2184,6 +2193,7 @@ 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 +2202,7 @@ 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 +2215,7 @@ 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 && fib_nh_saddr(net, fi, nexthop_nh, genid) == saddr)
 			nh_score += 2;
 		if (hash <= nh_upper_bound)
 			nh_score++;
-- 
2.25.1