linux-kernel - Re: [PATCH] media: pci: ttpci: av7110: avoid compiler optimization of reading data[0] in debiirq()

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Date:   Sun, 30 Aug 2020 09:30:36 +0100
From:   Sean Young <sean@...s.org>
To:     Jia-Ju Bai <baijiaju@...nghua.edu.cn>
Cc:     mchehab@...nel.org, pavel@....cz, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: pci: ttpci: av7110: avoid compiler optimization
 of reading data[0] in debiirq()

On Sun, Aug 30, 2020 at 04:20:42PM +0800, Jia-Ju Bai wrote:
> In debiirq(), data_0 stores the value of data[0], but it can be dropped
> by compiler optimization. Thus, data[0] is read through READ_ONCE().
> 
> Fixes: 6499a0db9b0f ("media: pci: ttpci: av7110: fix possible buffer overflow caused by bad DMA value in debiirq()")
> Reported-by: Pavel Machek <pavel@....cz>

Pavel reported that your patch was garbage, if you are trying to defend
against a malicious pci device. READ_ONCE() will not help here.

Sean

> Signed-off-by: Jia-Ju Bai <baijiaju@...nghua.edu.cn>
> ---
>  drivers/media/pci/ttpci/av7110.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/media/pci/ttpci/av7110.c b/drivers/media/pci/ttpci/av7110.c
> index bf36b1e22b63..f7d098d5b198 100644
> --- a/drivers/media/pci/ttpci/av7110.c
> +++ b/drivers/media/pci/ttpci/av7110.c
> @@ -406,7 +406,7 @@ static void debiirq(unsigned long cookie)
>  	case DATA_CI_GET:
>  	{
>  		u8 *data = av7110->debi_virt;
> -		u8 data_0 = data[0];
> +		u8 data_0 = READ_ONCE(data[0]);
>  
>  		if (data_0 < 2 && data[2] == 0xff) {
>  			int flags = 0;
> -- 
> 2.17.1