linux-kernel - Re: Faking MMIO ops? Fooling a driver

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Date:	Thu, 16 Jun 2011 23:47:02 +0200
From:	Rafał Miłecki <zajec5@...il.com>
To:	Pekka Paalanen <pq@....fi>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	linux-wireless@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Faking MMIO ops? Fooling a driver

W dniu 16 czerwca 2011 21:34 użytkownik Pekka Paalanen <pq@....fi> napisał:
> On Thu, 16 Jun 2011 21:19:04 +0200
> Rafał Miłecki <zajec5@...il.com> wrote:
>
>> W dniu 16 czerwca 2011 20:07 użytkownik Larry Finger
>> <Larry.Finger@...inger.net> napisał:
>> > On 06/16/2011 12:20 PM, Rafał Miłecki wrote:
>> >>
>> >> W dniu 16 czerwca 2011 16:44 użytkownik Rafał Miłecki
>> >> <zajec5@...il.com>  napisał:
>> >>>
>> >>> I analyze MMIO dumps of closed source driver and found such a
>> >>> place: W 2 3855.911536 9 0xb06003fc 0x810 0x0 0
>> >>> R 2 3855.911540 9 0xb06003fe 0x0 0x0 0
>> >>> W 2 3855.911541 9 0xb06003fe 0x0 0x0 0
>> >>>
>> >>> After translation:
>> >>>  phy_read(0x0810) ->  0x0000
>> >>> phy_write(0x0810)<- 0x0000
>> >>>
>> >>> So it's quite obvious, the driver is reading PHY register,
>> >>> masking it and writing masked value. Unfortunately from just
>> >>> looking at such place we can not guess the mask driver uses.
>> >>>
>> >>> I'd like to fake value read from 0xb06003fe to be 0xFFFF.
>> >>> Is there some ready method for doing such a trick?
>> >>>
>> >>> Dump comes from Kernel hacking → Tracers → MMIO and
>> >>> ndiswrapper.
>> >>
>> >> I can see values in MMIO trace struct are filled in
>> >> arch/x86/mm/mmio-mod.c in "pre" and "post". However still no
>> >> idea how to hack the returned value.
>
> If you want to do it that way, the idea is to overwrite
> the right CPU register in mmio-mod.c:post(). You would test for
> the address you want to mess with, and then "invert"
> get_ins_reg_val() to overwrite the register with your own value.

Good, idea thanks!


>> >> Should I try hacking read[bwl] instead? :|
>> >
>> > Probably. I do not see any way to trace and modify the results
>> > for a particular address without special code.
>>
>> Did you success with writing some special code? Following patch
>> does not seem to work for me:
>>
>>
>> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
>> index e0ffa3d..448e4ff 100644
>> --- a/include/asm-generic/io.h
>> +++ b/include/asm-generic/io.h
>> @@ -23,6 +23,8 @@
>>  #define mmiowb() do {} while (0)
>>  #endif
>>
>> +static int zajec = 0;
>> +
>>  /*****************************************************************************/
>>  /*
>>   * readX/writeX() are used to access memory mapped devices. On
>> some @@ -40,6 +42,11 @@ static inline u8 __raw_readb(const
>> volatile void __iomem *addr)
>>  #ifndef __raw_readw
>>  static inline u16 __raw_readw(const volatile void __iomem *addr)
>>  {
>> +     if (zajec++ < 10)
>> +             printk(KERN_INFO "[ZAJEC] %d\n", *addr);
>> +     if (*addr == 0xfaafc000)
>> +             printk(KERN_INFO "[ZAJEC] Bingo!\n");
>> +     //0x1381a8d8
>>       return *(const volatile u16 __force *) addr;
>>  }
>>  #endif
>>
>> Should I modify different readw? Path include/asm-generic/io.h
>> sounds sane to me, so I modified this one.
>
> Are you sure that code is ever used on your arch?
> For instance, it seems that arch/x86/include/asm/io.h
> defines a different __raw_readw.

You're right, I was using wrong file. Anyway it's too low level stuff
to use printk here. So thank you for the other suggested method!

-- 
Rafał
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