[PATCH net-next] netpoll: Fix race condition in netpoll_owner_active

[PATCH net-next] netpoll: Fix race condition in netpoll_owner_active

[Breno Leitao]

KCSAN detected a race condition in netpoll:

	BUG: KCSAN: data-race in net_rx_action / netpoll_send_skb
	write (marked) to 0xffff8881164168b0 of 4 bytes by interrupt on cpu 10:
	net_rx_action (./include/linux/netpoll.h:90 net/core/dev.c:6712 net/core/dev.c:6822)
<snip>
	read to 0xffff8881164168b0 of 4 bytes by task 1 on cpu 2:
	netpoll_send_skb (net/core/netpoll.c:319 net/core/netpoll.c:345 net/core/netpoll.c:393)
	netpoll_send_udp (net/core/netpoll.c:?)
<snip>
	value changed: 0x0000000a -> 0xffffffff

This happens because netpoll_owner_active() needs to check if the
current CPU is the owner of the lock, touching napi->poll_owner
non atomically. The ->poll_owner field contains the current CPU holding
the lock.

Use an atomic read to check if the poll owner is the current CPU.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 net/core/netpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 543007f159f9..55bcacf67df3 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -316,7 +316,7 @@ static int netpoll_owner_active(struct net_device *dev)
 	struct napi_struct *napi;
 
 	list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) {
-		if (napi->poll_owner == smp_processor_id())
+		if (READ_ONCE(napi->poll_owner) == smp_processor_id())
 			return 1;
 	}
 	return 0;
-- 
2.43.0

Re: [PATCH net-next] netpoll: Fix race condition in netpoll_owner_active

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 29 Apr 2024 03:04:33 -0700 you wrote:
> KCSAN detected a race condition in netpoll:
> 
> 	BUG: KCSAN: data-race in net_rx_action / netpoll_send_skb
> 	write (marked) to 0xffff8881164168b0 of 4 bytes by interrupt on cpu 10:
> 	net_rx_action (./include/linux/netpoll.h:90 net/core/dev.c:6712 net/core/dev.c:6822)
> <snip>
> 	read to 0xffff8881164168b0 of 4 bytes by task 1 on cpu 2:
> 	netpoll_send_skb (net/core/netpoll.c:319 net/core/netpoll.c:345 net/core/netpoll.c:393)
> 	netpoll_send_udp (net/core/netpoll.c:?)
> <snip>
> 	value changed: 0x0000000a -> 0xffffffff
> 
> [...]

Here is the summary with links:
  - [net-next] netpoll: Fix race condition in netpoll_owner_active
    https://git.kernel.org/netdev/net-next/c/c2e6a872bde9

You are awesome, thank you!
-- 
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