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

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

[Jia-Ju Bai]

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@ucw.cz>
Signed-off-by: Jia-Ju Bai <baijiaju@tsinghua.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

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

[Sean Young]

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@ucw.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@tsinghua.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

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

[Pavel Machek]

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On Sun 2020-08-30 09:30:36, Sean Young wrote:
> 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@ucw.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.

I would not use exactly those words, but agreed; we should have some
explanation that it is feasible to protect against malicious av7110
device, first.

Best regards,
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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