linux-kernel - Re: [PATCH net] NFC: hci: fix sleep in atomic context bugs in nfc_hci_hcp_message

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Date:   Mon, 16 May 2022 08:44:39 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Duoming Zhou <duoming@....edu.cn>, linux-kernel@...r.kernel.org
Cc:     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

On 16/05/2022 04:10, Duoming Zhou 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...



Best regards,
Krzysztof