wireguard: netlink: access device through ctx instead of peer

The previous commit fixed a bug that led to a NULL peer->device being
dereferenced. It's actually easier and faster performance-wise to
instead get the device from ctx->wg. This semantically makes more sense
too, since ctx->wg->peer_allowedips.seq is compared with
ctx->allowedips_seq, basing them both in ctx. This also acts as a
defence in depth provision against freed peers.

Cc: stable@vger.kernel.org
Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/wireguard/netlink.c

@@ -164,8 +164,8 @@ get_peer(struct wg_peer *peer, struct sk_buff *skb, struct dump_ctx *ctx)
 	if (!allowedips_node)
 		goto no_allowedips;
 	if (!ctx->allowedips_seq)
-		ctx->allowedips_seq = peer->device->peer_allowedips.seq;
-	else if (ctx->allowedips_seq != peer->device->peer_allowedips.seq)
+		ctx->allowedips_seq = ctx->wg->peer_allowedips.seq;
+	else if (ctx->allowedips_seq != ctx->wg->peer_allowedips.seq)
 		goto no_allowedips;
 
 	allowedips_nest = nla_nest_start(skb, WGPEER_A_ALLOWEDIPS);