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Date:   Mon, 16 May 2022 18:18:45 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org>
Cc:     linux-kernel@...r.kernel.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        gregkh@...uxfoundation.org, alexander.deucher@....com,
        broonie@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net] NFC: hci: fix sleep in atomic context bugs in
 nfc_hci_hcp_message_tx

Hello,

On Mon, 16 May 2022 08:44:39 +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@....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.

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.

Best regards,
Duoming Zhou

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