[PATCH net v2] xfrm: hold net_device reference under RCU in bundle creation

[Cen Zhang (Microsoft)]

From: "Cen Zhang (Microsoft Security FORGE Labs)" <blbllhy@xxxxxxxxx>

xfrm_bundle_create() and xfrm_create_dummy_bundle() read dst->dev into
a local pointer without taking a device reference, then pass it to
xfrm_fill_dst(). A concurrent RTM_DELLINK replaces dst->dev via
dst_dev_put() and frees the old net_device, causing a use-after-free
when xfrm6_fill_dst() later dereferences the stale dev pointer.

  BUG: KASAN: slab-use-after-free in xfrm6_fill_dst+0x82c/0x860
  (net/ipv6/xfrm6_policy.c:86 netdev_hold())
  Read of size 8 at addr ffff8880142fe588 by task exploit/153
  Call Trace:
   xfrm6_fill_dst+0x82c/0x860
   xfrm_resolve_and_create_bundle+0x21d4/0x2bd0
   xfrm_lookup_with_ifid+0x485/0x1640
   ip6_dst_lookup_flow+0x19b/0x1e0
   udpv6_sendmsg+0x1443/0x2dd0

Fix this by reading dst->dev via dst_dev_rcu() and keeping the RCU
read-side critical section active until xfrm_fill_dst() has taken the
required device references.

Fixes: 25ee3286dcbc ("[IPSEC]: Merge common code into xfrm_bundle_create")
Fixes: a0073fe18e71 ("xfrm: Add a state resolution packet queue")
Suggested-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
Reported-by: Xiang Mei (Microsoft) <xmei5@xxxxxxx>
Link: https://lore.kernel.org/all/20260820200245.44312-1-blbllhy@xxxxxxxxx/
Cc: AutonomousCodeSecurity@xxxxxxxxxxxxx
Assisted-by: GitHub-Copilot:claude-opus-4.6
Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <blbllhy@xxxxxxxxx>
---
Changes in v2:
- Keep the RCU read-side critical section active through xfrm_fill_dst()
  instead of taking a temporary device reference, as suggested by
  Steffen.

 net/xfrm/xfrm_policy.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 932a313b9460..513c9f228334 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2770,9 +2770,12 @@ static struct dst_entry *xfrm_bundle_create(struct xfrm_policy *policy,
 	xdst0->path = dst;
 
 	err = -ENODEV;
-	dev = dst->dev;
-	if (!dev)
+	rcu_read_lock();
+	dev = dst_dev_rcu(dst);
+	if (!dev) {
+		rcu_read_unlock();
 		goto free_dst;
+	}
 
 	xfrm_init_path(xdst0, dst, nfheader_len);
 	xfrm_init_pmtu(bundle, nx);
@@ -2780,8 +2783,10 @@ static struct dst_entry *xfrm_bundle_create(struct xfrm_policy *policy,
 	for (xdst_prev = xdst0; xdst_prev != (struct xfrm_dst *)dst;
 	     xdst_prev = (struct xfrm_dst *) xfrm_dst_child(&xdst_prev->u.dst)) {
 		err = xfrm_fill_dst(xdst_prev, dev, fl);
-		if (err)
+		if (err) {
+			rcu_read_unlock();
 			goto free_dst;
+		}
 
 		xdst_prev->u.dst.header_len = header_len;
 		xdst_prev->u.dst.trailer_len = trailer_len;
@@ -2789,6 +2794,7 @@ static struct dst_entry *xfrm_bundle_create(struct xfrm_policy *policy,
 		trailer_len -= xdst_prev->u.dst.xfrm->props.trailer_len;
 	}
 
+	rcu_read_unlock();
 	return &xdst0->u.dst;
 
 put_states:
@@ -3058,11 +3064,15 @@ static struct xfrm_dst *xfrm_create_dummy_bundle(struct net *net,
 	xfrm_init_path((struct xfrm_dst *)dst1, dst, 0);
 
 	err = -ENODEV;
-	dev = dst->dev;
-	if (!dev)
+	rcu_read_lock();
+	dev = dst_dev_rcu(dst);
+	if (!dev) {
+		rcu_read_unlock();
 		goto free_dst;
+	}
 
 	err = xfrm_fill_dst(xdst, dev, fl);
+	rcu_read_unlock();
 	if (err)
 		goto free_dst;
 
-- 
2.55.0