netpoll: fix IPv6 local-address corruption

[ Upstream commit 3bc179b ]

netpoll_setup() decides whether to auto-populate the local source
address by testing np->local_ip.ip, which only inspects the first 4
bytes of the union inet_addr storage.

For an IPv6 netpoll whose caller-supplied local address has a zero
high-32 bits (::1, ::<suffix>, IPv4-mapped ::ffff:a.b.c.d, etc.), this
misdetects the address as unset (which they are not, but the first
4 bytes are empty), calls netpoll_take_ipv6() and overwrites it with
whatever matching link-local/global address the device happens to expose
first.

Introduce a helper netpoll_local_ip_unset() that picks the correct
family-aware test (ipv6_addr_any() for IPv6, !.ip for IPv4) and use it
from netpoll_setup().

Reproducer is something like:

  echo "::2" > local_ip
  echo 1     > enabled
  cat local_ip
  # before this fix: 2001:db8::1   (caller-supplied ::2 was clobbered)
  # after  this fix: ::2

Fixes: b7394d2 ("netpoll: prepare for ipv6")
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260424-netpoll_fix-v1-1-3a55348c625f@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: leitao

net/core/netpoll.c

@@ -750,6 +750,23 @@ static int netpoll_take_ipv4(struct netpoll *np, struct net_device *ndev)
 	return 0;
 }
 
+/*
+ * Test whether the caller left np->local_ip unset, so that
+ * netpoll_setup() should auto-populate it from the egress device.
+ *
+ * np->local_ip is a union of __be32 (IPv4) and struct in6_addr (IPv6),
+ * so an IPv6 address whose first 4 bytes are zero (e.g. ::1, ::2,
+ * IPv4-mapped ::ffff:a.b.c.d) must not be tested via the IPv4 arm —
+ * doing so would misclassify a caller-supplied address as unset and
+ * silently overwrite it with whatever address the device exposes.
+ */
+static bool netpoll_local_ip_unset(const struct netpoll *np)
+{
+	if (np->ipv6)
+		return ipv6_addr_any(&np->local_ip.in6);
+	return !np->local_ip.ip;
+}
+
 int netpoll_setup(struct netpoll *np)
 {
 	struct net *net = current->nsproxy->net_ns;
@@ -793,7 +810,7 @@ int netpoll_setup(struct netpoll *np)
 		rtnl_lock();
 	}
 
-	if (!np->local_ip.ip) {
+	if (netpoll_local_ip_unset(np)) {
 		if (!np->ipv6) {
 			err = netpoll_take_ipv4(np, ndev);
 			if (err)