Re: [net v3] net: wwan: t7xx: Fix napi rx poll issue

Larysa Zaremba
2025-06-03T09:37:28Z

On Fri, May 30, 2025 at 11:16:48AM +0800, Jinjian Song wrote:
> When driver handles the napi rx polling requests, the netdev might
> have been released by the dellink logic triggered by the disconnect
> operation on user plane. However, in the logic of processing skb in
> polling, an invalid netdev is still being used, which causes a panic.
>
> BUG: kernel NULL pointer dereference, address: 00000000000000f1
> Oops: 0000 [#1] PREEMPT SMP NOPTI
> RIP: 0010:dev_gro_receive+0x3a/0x620
> [...]
> Call Trace:
> <IRQ>
> ? __die_body+0x68/0xb0
> ? page_fault_oops+0x379/0x3e0
> ? exc_page_fault+0x4f/0xa0
> ? asm_exc_page_fault+0x22/0x30
> ? __pfx_t7xx_ccmni_recv_skb+0x10/0x10 [mtk_t7xx (HASH:1400 7)]
> ? dev_gro_receive+0x3a/0x620
> napi_gro_receive+0xad/0x170
> t7xx_ccmni_recv_skb+0x48/0x70 [mtk_t7xx (HASH:1400 7)]
> t7xx_dpmaif_napi_rx_poll+0x590/0x800 [mtk_t7xx (HASH:1400 7)]
> net_rx_action+0x103/0x470
> irq_exit_rcu+0x13a/0x310
> sysvec_apic_timer_interrupt+0x56/0x90
> </IRQ>
>
> Fixes: 5545b7b9f294 ("net: wwan: t7xx: Add NAPI support")
> Signed-off-by: Jinjian Song <jinjian.song@fibocom.com>
> ---
> v3:
> * Only Use READ_ONCE/WRITE_ONCE when the lock protecting ctlb->ccmni_inst
> is not held.

What do you mean by "lock protecting ctlb->ccmni_inst"? Please specify.

> ---
> drivers/net/wwan/t7xx/t7xx_netdev.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/wwan/t7xx/t7xx_netdev.c b/drivers/net/wwan/t7xx/t7xx_netdev.c
> index 91fa082e9cab..fc0a7cb181df 100644
> --- a/drivers/net/wwan/t7xx/t7xx_netdev.c
> +++ b/drivers/net/wwan/t7xx/t7xx_netdev.c
> @@ -302,7 +302,7 @@ static int t7xx_ccmni_wwan_newlink(void *ctxt, struct net_device *dev, u32 if_id
> ccmni->ctlb = ctlb;
> ccmni->dev = dev;
> atomic_set(&ccmni->usage, 0);
> - ctlb->ccmni_inst[if_id] = ccmni;
> + WRITE_ONCE(ctlb->ccmni_inst[if_id], ccmni);
>
> ret = register_netdevice(dev);
> if (ret)
> @@ -324,6 +324,7 @@ static void t7xx_ccmni_wwan_dellink(void *ctxt, struct net_device *dev, struct l
> if (WARN_ON(ctlb->ccmni_inst[if_id] != ccmni))
> return;
>
> + WRITE_ONCE(ctlb->ccmni_inst[if_id], NULL);
> unregister_netdevice(dev);
> }
>
> @@ -419,7 +420,7 @@ static void t7xx_ccmni_recv_skb(struct t7xx_ccmni_ctrl *ccmni_ctlb, struct sk_bu
>
> skb_cb = T7XX_SKB_CB(skb);
> netif_id = skb_cb->netif_idx;
> - ccmni = ccmni_ctlb->ccmni_inst[netif_id];
> + ccmni = READ_ONCE(ccmni_ctlb->ccmni_inst[netif_id]);
> if (!ccmni) {
> dev_kfree_skb(skb);
> return;
> @@ -441,7 +442,7 @@ static void t7xx_ccmni_recv_skb(struct t7xx_ccmni_ctrl *ccmni_ctlb, struct sk_bu
>
> static void t7xx_ccmni_queue_tx_irq_notify(struct t7xx_ccmni_ctrl *ctlb, int qno)
> {
> - struct t7xx_ccmni *ccmni = ctlb->ccmni_inst[0];
> + struct t7xx_ccmni *ccmni = READ_ONCE(ctlb->ccmni_inst[0]);
> struct netdev_queue *net_queue;
>

You do not seem to check if ccmni is NULL here, so given ctlb->ccmni_inst[0] is
not being hot-swapped, I guess that there are some guarantees of it not being
NULL at this moment, so I would drop READ_ONCE here.

> if (netif_running(ccmni->dev) && atomic_read(&ccmni->usage) > 0) {
> @@ -453,7 +454,7 @@ static void t7xx_ccmni_queue_tx_irq_notify(struct t7xx_ccmni_ctrl *ctlb, int qno
>
> static void t7xx_ccmni_queue_tx_full_notify(struct t7xx_ccmni_ctrl *ctlb, int qno)
> {
> - struct t7xx_ccmni *ccmni = ctlb->ccmni_inst[0];
> + struct t7xx_ccmni *ccmni = READ_ONCE(ctlb->ccmni_inst[0]);
> struct netdev_queue *net_queue;
>

Same as above, either READ_ONCE is not needed or NULL check is required.

> if (atomic_read(&ccmni->usage) > 0) {
> @@ -471,7 +472,7 @@ static void t7xx_ccmni_queue_state_notify(struct t7xx_pci_dev *t7xx_dev,
> if (ctlb->md_sta != MD_STATE_READY)
> return;
>
> - if (!ctlb->ccmni_inst[0]) {
> + if (!READ_ONCE(ctlb->ccmni_inst[0])) {
> dev_warn(&t7xx_dev->pdev->dev, "No netdev registered yet\n");
> return;
> }
> --
> 2.34.1
>
>