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

[Linkui Xiao]

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. Annotate it as well so that the value cannot be torn or
reloaded while the per-nexthop scores are being computed.

Fixes: 195374d89368 ("ipv4: fib: annotate races around nh->nh_saddr_genid and nh->nh_saddr")
Signed-off-by: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
---
 net/ipv4/fib_semantics.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 7a362f2e2c2b..885c6fae5232 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -2204,7 +2204,7 @@ 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 && READ_ONCE(nexthop_nh->nh_saddr) == saddr)
 			nh_score += 2;
 		if (hash <= nh_upper_bound)
 			nh_score++;
-- 
2.25.1