Subject: [PATCH 20/21] net: disable netpoll on fresh napis

From: Jakub Kicinski <[email-masked]>
Date: Wed, 26 Aug 2020 12:40:06 -0700

[ Upstream commit 96e97bc07e90f175a8980a22827faf702ca4cb30 ]

napi_disable() makes sure to set the NAPI_STATE_NPSVC bit to prevent
netpoll from accessing rings before init is complete. However, the
same is not done for fresh napi instances in netif_napi_add(),
even though we expect NAPI instances to be added as disabled.

This causes crashes during driver reconfiguration (enabling XDP,
changing the channel count) - if there is any printk() after
netif_napi_add() but before napi_enable().

To ensure memory ordering is correct we need to use RCU accessors.

Reported-by: Rob Sherwood <[email-masked]>
Fixes: 2d8bff12699a ("netpoll: Close race condition between poll_one_napi and napi_disable")
Signed-off-by: Jakub Kicinski <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/core/dev.c     | 3 ++-
 net/core/netpoll.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 25858f1f67cf..51ddc5a88cc2 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6230,12 +6230,13 @@ void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
 		netdev_err_once(dev, "%s() called with weight %d\n", __func__,
 				weight);
 	napi->weight = weight;
-	list_add(&napi->dev_list, &dev->napi_list);
 	napi->dev = dev;
 #ifdef CONFIG_NETPOLL
 	napi->poll_owner = -1;
 #endif
 	set_bit(NAPI_STATE_SCHED, &napi->state);
+	set_bit(NAPI_STATE_NPSVC, &napi->state);
+	list_add_rcu(&napi->dev_list, &dev->napi_list);
 	napi_hash_add(napi);
 }
 EXPORT_SYMBOL(netif_napi_add);
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 849380a622ef..cb67d36f3adb 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -161,7 +161,7 @@ static void poll_napi(struct net_device *dev)
 	struct napi_struct *napi;
 	int cpu = smp_processor_id();
 
-	list_for_each_entry(napi, &dev->napi_list, dev_list) {
+	list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) {
 		if (cmpxchg(&napi->poll_owner, -1, cpu) == -1) {
 			poll_one_napi(napi);
 			smp_store_release(&napi->poll_owner, -1);
-- 
2.26.2