[PATCH 5.10/5.15] xfrm6: clear dst.dev on error to avoid double netdev_put in xfrm6_fill_dst()

[Mikhail Dmitrichenko <mdmitrichenko@astralinux.ru>]

From: Xiang Mei (Microsoft) <xmei5@asu.edu>

commit 136992de9bb91871084ae52d172610541c76e4d2 upstream.

On the error path where in6_dev_get(dev) returns NULL, xfrm6_fill_dst()
releases the device reference with netdev_put() but leaves
xdst->u.dst.dev set. dst_destroy() later calls netdev_put(dst->dev)
again, so the same net_device reference is released twice, underflowing
its refcount (ref_tracker WARNING + "unregister_netdevice: waiting for
<dev> to become free").

Clear xdst->u.dst.dev after the netdev_put(), the same way the XFRM
device-offload paths xfrm_dev_state_add() and xfrm_dev_policy_add() in
net/xfrm/xfrm_device.c NULL ->dev when releasing the reference on error.

  ref_tracker: reference already released.
  ref_tracker: allocated in:
   xfrm6_fill_dst (net/ipv6/xfrm6_policy.c:86)
   ...
   udpv6_sendmsg (net/ipv6/udp.c:1696)
   ...
  ref_tracker: freed in:
   xfrm6_fill_dst (net/ipv6/xfrm6_policy.c:90)
   ...
  WARNING: lib/ref_tracker.c:322 at ref_tracker_free+0x58b/0x780
   dst_destroy (net/core/dst.c:115)
   rcu_core
   handle_softirqs
   ...

Fixes: 84c4a9dfbf43 ("xfrm6: release dev before returning error")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
[ mdmitrichenko: 5.10 and 5.15 predate the net device refcount tracker,
  so the error path there drops the reference with plain dev_put()
  rather than netdev_put(); the fix itself is the same. ]
Signed-off-by: Mikhail Dmitrichenko <mdmitrichenko@astralinux.ru>
---
Backport fix for CVE-2026-64580
 net/ipv6/xfrm6_policy.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index b504eeb59224..95dd8acd45ad 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -88,6 +88,7 @@ static int xfrm6_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
 	xdst->u.rt6.rt6i_idev = in6_dev_get(dev);
 	if (!xdst->u.rt6.rt6i_idev) {
 		dev_put(dev);
+		xdst->u.dst.dev = NULL;
 		return -ENODEV;
 	}
 
-- 
2.43.0