[WireGuard] [PATCH v3] ip6_output: ensure flow saddr actually belongs to device

["Jason A. Donenfeld" <Jason@zx2c4.com>]

This puts the IPv6 routing functions in parity with the IPv4 routing
functions. Namely, we now check in v6 that if a flowi6 requests an
saddr, the returned dst actually corresponds to a net device that has
that saddr. This mirrors the v4 logic with __ip_dev_find in
__ip_route_output_key_hash. In the event that the returned dst is not
for a dst with a dev that has the saddr, we return -EINVAL, just like
v4; this makes it easy to use the same error handlers for both cases.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: David Ahern <dsa@cumulusnetworks.com>
---
Changes from v2:
    It turns out ipv6_chk_addr already has the device enumeration
    logic that we need by simply passing NULL.

 net/ipv6/ip6_output.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 6001e78..b3b5cb6 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -926,6 +926,10 @@ static int ip6_dst_lookup_tail(struct net *net, const struct sock *sk,
 	int err;
 	int flags = 0;
 
+	if (!ipv6_addr_any(&fl6->saddr) &&
+	    !ipv6_chk_addr(net, &fl6->saddr, NULL, 1))
+		return -EINVAL;
+
 	/* The correct way to handle this would be to do
 	 * ip6_route_get_saddr, and then ip6_route_output; however,
 	 * the route-specific preferred source forces the
-- 
2.10.2