[PATCH net] bonding: fix skb_under_panic in bond_ns_send() over stacked VLANs

["Xiang Mei (Microsoft)" <xmei5@asu.edu>]

bond_ns_send() builds an IPv6 Neighbor Solicitation with
ndisc_ns_create(), which reserves exactly LL_RESERVED_SPACE(dev) +
sizeof(struct ipv6hdr) of headroom for the later ip6_nd_hdr() push.
bond_handle_vlan() then inserts the collected VLAN tags into the skb;
each inner tag consumes VLAN_HLEN of that headroom via skb_push(). With
enough stacked VLAN devices between the bond and the ns_ip6_target, the
reserved IPv6 headroom is exhausted, so the subsequent
ndisc_send_skb() -> ip6_nd_hdr() -> skb_push(sizeof(struct ipv6hdr))
underflows past skb->head and hits skb_under_panic().

Restore the required headroom with skb_cow_head() after VLAN insertion
and before handing the skb to ndisc_send_skb(); drop the probe on
allocation failure. For paths that did not exhaust the headroom this is
a no-op, so previously working configurations are unaffected.

  skbuff: skb_under_panic: len:84 put:40 head:... data:... tail:0x50 end:0x180 dev:veth0
  kernel BUG at net/core/skbuff.c:214!
  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
  Workqueue: bond0 bond_arp_monitor
  RIP: 0010:skb_panic+0x142/0x230
  Call Trace:
   skb_push (net/core/skbuff.c:224)
   ndisc_send_skb (net/ipv6/ndisc.c:454 net/ipv6/ndisc.c:506)
   bond_ns_send (drivers/net/bonding/bond_main.c:3255)
   bond_ns_send_all (drivers/net/bonding/bond_main.c:3313)
   bond_arp_monitor (drivers/net/bonding/bond_main.c:3458)
   process_one_work (kernel/workqueue.c:3322)
   worker_thread (kernel/workqueue.c:3405)
   kthread (kernel/kthread.c:436)
  Kernel panic - not syncing: Fatal exception

Fixes: 4e24be018eb9 ("bonding: add new parameter ns_targets")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
---
 drivers/net/bonding/bond_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index e044fc733b8c..3ac3418c9498 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -3251,6 +3251,10 @@ static void bond_ns_send(struct slave *slave, const struct in6_addr *daddr,
 
 	addrconf_addr_solict_mult(daddr, &mcaddr);
 	if (bond_handle_vlan(slave, tags, skb)) {
+		if (skb_cow_head(skb, sizeof(struct ipv6hdr))) {
+			kfree_skb(skb);
+			return;
+		}
 		slave_update_last_tx(slave);
 		ndisc_send_skb(skb, &mcaddr, saddr);
 	}
-- 
2.43.0