linux-kernel - Re: [PATCH] ARM: fix alignement of __bug

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Date:	Thu, 10 Sep 2015 21:01:41 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: fix alignement of __bug_table section entries

Robert Jarzmik <robert.jarzmik@...e.fr> writes:

> Russell King - ARM Linux <linux@....linux.org.uk> writes:
>
>> On Tue, Sep 08, 2015 at 07:01:00PM +0200, Robert Jarzmik wrote:
>>> Russell King - ARM Linux <linux@....linux.org.uk> writes:
>> At the point we call into this code, the DACR should be 0x75, which
>> should allow us to read the instruction at 0xbf00202c.  But this is
>> failing with a permission error - which it would do if it thought
>> the kernel domain was in manager mode (iow, 0x55).
>
> Okay Russell, I have a good idea what's happening now. Basically, it boils down
> to compiler optimization of get_domain() which is called twice (set_fs() ->
> modify_domain() -> get_domain()). See the piece in [1] for a more complete
> explanation.
>
> I still haven't finished my work, as I need to disassemble the do_alignment()
And I have the proof of gcc optimization, which I'll add to the commit message
if you want :
00000728 <do_alignment>:
     ...
     770:	ee134f10 	mrc	15, 0, r4, cr3, cr0, {0}
     ... no r4 or mrc/mcr usage
     788:	e3842030 	orr	r2, r4, #48	; 0x30
     ... no r2/r4 or mrc/mcr usage
     794:	ee032f10 	mcr	15, 0, r2, cr3, cr0, {0}
     798:	ee07cf95 	mcr	15, 0, ip, cr7, cr5, {4}
     ... no r4 or mrc/mcr usage
     7ac:	e3c4300c 	bic	r3, r4, #12
     7b0:	e3833004 	orr	r3, r3, #4
     7b4:	ee033f10 	mcr	15, 0, r3, cr3, cr0, {0}
     ... no mrc/mcr usage
     7cc:	ebfffffe 	bl	0 <arm_copy_from_user>

Here, we have in probe_kernel_address() in do_alignment():
 - @770 : r4 = DACR
 - @794 : DACR = r4 | 0x30
 - @7b4 : DACR = (r4 & 0x0c) | 0x04 => the 0x30 is lost !!!

I'll send my patch to the mailing list tomorrow, as well as the other one to
align the __bug_table session.

Cheers.

--
Robert
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