linux-kernel - Re: [PATCH AUTOSEL 4.19 08/38] media: pci: ttpci: av7110: fix possible buffer overflow caused by bad DMA value in debiirq()

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Message-ID: <20200829121020.GA20944@duo.ucw.cz>
Date:   Sat, 29 Aug 2020 14:10:20 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jia-Ju Bai <baijiaju@...nghua.edu.cn>,
        Sean Young <sean@...s.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.19 08/38] media: pci: ttpci: av7110: fix
 possible buffer overflow caused by bad DMA value in debiirq()

Hi!

> The value av7110->debi_virt is stored in DMA memory, and it is assigned
> to data, and thus data[0] can be modified at any time by malicious
> hardware. In this case, "if (data[0] < 2)" can be passed, but then
> data[0] can be changed into a large number, which may cause buffer
> overflow when the code "av7110->ci_slot[data[0]]" is used.
> 
> To fix this possible bug, data[0] is assigned to a local variable, which
> replaces the use of data[0].

I'm pretty sure hardware capable of manipulating memory can work
around any such checks, but...

> +++ b/drivers/media/pci/ttpci/av7110.c
> @@ -424,14 +424,15 @@ static void debiirq(unsigned long cookie)
>  	case DATA_CI_GET:
>  	{
>  		u8 *data = av7110->debi_virt;
> +		u8 data_0 = data[0];
>  
> -		if ((data[0] < 2) && data[2] == 0xff) {
> +		if (data_0 < 2 && data[2] == 0xff) {
>  			int flags = 0;
>  			if (data[5] > 0)
>  				flags |= CA_CI_MODULE_PRESENT;
>  			if (data[5] > 5)
>  				flags |= CA_CI_MODULE_READY;
> -			av7110->ci_slot[data[0]].flags = flags;
> +			av7110->ci_slot[data_0].flags = flags;

This does not even do what it says. Compiler is still free to access
data[0] multiple times. It needs READ_ONCE() to be effective.

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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