Re: [PATCH net] NFC: hci: fix sleep in atomic context bugs in nfc_hci_hcp_message_tx

[duoming@zju.edu.cn]

Hello,

On Mon, 16 May 2022 12:43:07 +0200 Krzysztof wrote:

> >>> There are sleep in atomic context bugs when the request to secure
> >>> element of st21nfca is timeout. The root cause is that kzalloc and
> >>> alloc_skb with GFP_KERNEL parameter is called in st21nfca_se_wt_timeout
> >>> which is a timer handler. The call tree shows the execution paths that
> >>> could lead to bugs:
> >>>
> >>>    (Interrupt context)
> >>> st21nfca_se_wt_timeout
> >>>   nfc_hci_send_event
> >>>     nfc_hci_hcp_message_tx
> >>>       kzalloc(..., GFP_KERNEL) //may sleep
> >>>       alloc_skb(..., GFP_KERNEL) //may sleep
> >>>
> >>> This patch changes allocation mode of kzalloc and alloc_skb from
> >>> GFP_KERNEL to GFP_ATOMIC in order to prevent atomic context from
> >>> sleeping. The GFP_ATOMIC flag makes memory allocation operation
> >>> could be used in atomic context.
> >>>
> >>> Fixes: 8b8d2e08bf0d ("NFC: HCI support")
> >>> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> >>> ---
> >>>  net/nfc/hci/hcp.c | 4 ++--
> >>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/net/nfc/hci/hcp.c b/net/nfc/hci/hcp.c
> >>> index 05c60988f59..1caf9c2086f 100644
> >>> --- a/net/nfc/hci/hcp.c
> >>> +++ b/net/nfc/hci/hcp.c
> >>> @@ -30,7 +30,7 @@ int nfc_hci_hcp_message_tx(struct nfc_hci_dev *hdev, u8 pipe,
> >>>  	int hci_len, err;
> >>>  	bool firstfrag = true;
> >>>  
> >>> -	cmd = kzalloc(sizeof(struct hci_msg), GFP_KERNEL);
> >>> +	cmd = kzalloc(sizeof(*cmd), GFP_ATOMIC);
> >>
> >> No, this does not look correct. This function can sleep, so it can use
> >> GFP_KERNEL. Please just look at the function before replacing any flags...
> > 
> > I am sorry, I don`t understand why nfc_hci_hcp_message_tx() can sleep.
> 
> Why? because in line 93 it uses a mutex (which is a sleeping primitve).
> 
> > 
> > I think st21nfca_se_wt_timeout() is a timer handler, which is in interrupt context.
> > The call tree shows the execution paths that could lead to bugs:
> > 
> > st21nfca_hci_se_io()
> >   mod_timer(&info->se_info.bwi_timer,...)
> >     st21nfca_se_wt_timeout()  //timer handler, interrupt context
> >       nfc_hci_send_event()
> >         nfc_hci_hcp_message_tx()
> >           kzalloc(..., GFP_KERNEL) //may sleep
> >           alloc_skb(..., GFP_KERNEL) //may sleep
> > 
> > What`s more, I think the "mutex_lock(&hdev->msg_tx_mutex)" called by nfc_hci_hcp_message_tx()
> > is also improper.
> > 
> > Please correct me, If you think I am wrong. Thanks for your time.
> 
> Your patch is not correct in current semantics of this function. This
> function can sleep (because it uses a mutex), so the mistake is rather
> calling it from interrupt context.

We have to call nfc_hci_send_event() in st21nfca_se_wt_timeout(), because we need to send 
a reset request as recovery procedure. I think replace GFP_KERNEL to GFP_ATOMIC and replace
mutex_lock to spin_lock in nfc_hci_hcp_message_tx() is better.

What's more, in order to synchronize with other functions related with hci message TX, 
We need to replace the mutex_lock(&hdev->msg_tx_mutex) to spin_lock in other functions
as well. I sent "patch v2" just now.

Best regards,
Duoming Zhou