Re: Faking MMIO ops? Fooling a driver

[Pekka Paalanen <pq@iki.fi>]

On Thu, 16 Jun 2011 21:19:04 +0200
Rafał Miłecki <zajec5@gmail.com> wrote:

> W dniu 16 czerwca 2011 20:07 użytkownik Larry Finger
> <Larry.Finger@lwfinger.net> napisał:
> > On 06/16/2011 12:20 PM, Rafał Miłecki wrote:
> >>
> >> W dniu 16 czerwca 2011 16:44 użytkownik Rafał Miłecki
> >> <zajec5@gmail.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.

> >> 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.


Cheers.

-- 
Pekka Paalanen
http://www.iki.fi/pq/