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

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Date:	Thu, 16 Jun 2011 21:19:04 +0200
From:	Rafał Miłecki <zajec5@...il.com>
To:	Larry Finger <Larry.Finger@...inger.net>
Cc:	linux-wireless@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pekka Paalanen <pq@....fi>
Subject: Re: Faking MMIO ops? Fooling a driver

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.
>>
>> 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.


> FYI, my reference driver for reverse engineering has no instance of a
> read/modify/write for PHY register 0x810. Is the code in question for a PHY
> type > 6?

This is for PHY type 7, AKA "HT".

-- 
Rafał
--
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