Re: [PATCH net v3] gtp: serialize PDP context updates

PDP contexts can be deleted through GTP_CMD_DELPDP or while the GTP
network device is being unregistered. The latter is serialized by RTNL,
but the generic-netlink delete path only holds RCU.

Running both paths concurrently can therefore make both paths delete the
same PDP context. The issue was found through static analysis and
reproduced on a KASAN-enabled kernel by a two-thread program
racing GTP_CMD_DELPDP against RTM_DELLINK:

  Oops: general protection fault, probably for non-canonical address
  KASAN: maybe wild-memory-access in range
         [0xdead000000000120-0xdead000000000127]
  RIP: gtp_genl_del_pdp+0x1c1/0x420 [gtp]
  RBP: dead000000000122

The second deletion dereferenced the poisoned hlist pprev pointer.

Serialize PDP context updates with a mutex shared by gtp_pdp_add(),
gtp_genl_del_pdp(), and gtp_dellink().

Fixes: 459aa660eb1d ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Assisted-by: Codex:gpt-5
Signed-off-by: Qing Ming <redacted>
---
v3:
- Rebase onto net/main.

v2: https://lore.kernel.org/netdev/20260808124322.5800-1-a0yami@mailbox.org/ (local)
- Use a dedicated mutex instead of RTNL.
- Protect PDP add, delete, and link teardown with the same mutex.

 drivers/net/gtp.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 9a12cc53da00..8ee516debeed 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c

@@ -12,6 +12,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/skbuff.h>
 #include <linux/udp.h>
 #include <linux/rculist.h>

@@ -108,6 +109,7 @@ struct gtp_net {
 };
 
 static u32 gtp_h_initval;
+static DEFINE_MUTEX(gtp_pdp_lock);
 
 static struct genl_family gtp_genl_family;
 

@@ -1555,9 +1557,11 @@ static void gtp_dellink(struct net_device *dev, struct list_head *head)
 	struct pdp_ctx *pctx;
 	int i;
 
+	mutex_lock(&gtp_pdp_lock);
 	for (i = 0; i < gtp->hash_size; i++)
 		hlist_for_each_entry_safe(pctx, next, &gtp->tid_hash[i], hlist_tid)
 			pdp_context_delete(pctx);
+	mutex_unlock(&gtp_pdp_lock);
 
 	list_del(&gtp->list);
 	unregister_netdevice_queue(dev, head);

@@ -1833,6 +1837,8 @@ static struct pdp_ctx *gtp_pdp_add(struct gtp_dev *gtp, struct sock *sk,
 	__be32 ms_addr;
 	int family;
 
+	guard(mutex)(&gtp_pdp_lock);
+