Re: [PATCH net v2] gtp: add synchronize_net() in gtp_newlink() error path to prevent use-after-free

gtp_newlink()'s error path frees tid_hash and addr_hash without
waiting for an RCU grace period after clearing sk_user_data. A
concurrent gtp_encap_recv() in softirq may still hold the gtp_dev
pointer obtained via rcu_dereference_sk_user_data() and access the
freed memory.

  BUG: KASAN: slab-use-after-free in gtp0_pdp_find+0x1f6/0x200 (gtp.c:152)
  Call Trace:
   <IRQ>
   gtp0_pdp_find+0x1f6/0x200
   gtp_encap_recv+0x527/0x24b0
   udp_queue_rcv_one_skb+0x75f/0xc10

Add synchronize_net() before the kfree calls in out_hashtable, which
covers all error paths from both gtp_encap_enable() and
gtp_create_sockets().

Fixes: 459aa660eb1d8ce6 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Xiang Mei (Microsoft) <redacted>
Reported-by: Cen Zhang (Microsoft) <redacted>
Signed-off-by: Cen Zhang (Microsoft) <redacted>
---
v2: Add the net tree subject prefix.

 drivers/net/gtp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 9a12cc53da00..f11bcc0c0ac4 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c

@@ -1543,6 +1543,8 @@ static int gtp_newlink(struct net_device *dev,
 out_encap:
 	gtp_encap_disable(gtp);
 out_hashtable:
+	/* Wait for RCU readers that may still reference this gtp_dev. */
+	synchronize_net();
 	kfree(gtp->addr_hash);
 	kfree(gtp->tid_hash);
 	return err;