[PATCH 0/3] Make kexec work with CONFIG_DEBUG_RODATA enabled

[Kees Cook]

On Wed, Apr 30, 2014 at 6:19 AM, Nikolay Borisov
<Nikolay.Borisov at arm.com> wrote:
>> From: Will Deacon [mailto:will.deacon at arm.com]
>> Hi Nikolay,
>>
>> On Mon, Apr 28, 2014 at 10:31:44AM +0100, Nikolay Borisov wrote:
>> > This patch makes kexec work on a kernel that has its .text section
>> set to read
>> > only. The main reason of doing it is because there is a patch in the
>> making
>> > (http://lists.infradead.org/pipermail/linux-arm-kernel/2014-
>> April/244779.html)
>> > which aims to make it possible to flag the kernel's code section as
>> read-only,
>> > rendering kexec inoperable.
>> >
>> > The first patch does the actual functional changes while the 2nd one
>> is
>> > optional and is only to be used in case you have already applied the
>> aforementioned
>> > CONFIG_DEBUG_RODATA patch
>> >
>> > Patch 3 is also optional and tries to make the assembly code a bit
>> more "sane"
>> > by introducing a .struct to describe the parameters for the kernel
>> relocation
>> > stub.
>> >
>> > Those patches have been tested on linux 3.14 on THUMB-2/non-THUMB-2
>> host
>> > kernels.
>> >
>> > Patch 1 and 3 apply cleanly on 3.15-rc3 and Patch 2 applies cleanly,
>> provided
>> > that Kees' patch is applied as well.
>> >
>> > Kees also managed to test that on a QEMU 2.0 based machine.
>>
>> Wouldn't it be far simpler to set_kernel_text_rw() on a kexec? At this
>> point, we've committed to replacing our image, so whether we can write
>> to the .text of the old image before we clobber it entirely seems somewhat
>> moot to me.
>>
>
> We are poking one of the values in the prepare stage so doing set_kernel_text_rw() there
> won't be very clean solution. However, I agree that if the values being poked into the prep

set_kernel_text_rw() is also mm-specific, since the PMD sections are
copied per-mm. I'm not sure if that's as much of a problem here since
the prepare looks to be all in the same thread, but if we can avoid
set_kernel_text_rw() that'll probably make things less surprising if
changes are made later on.

-Kees

> stage is stashed in a static variable and then in the kexec_execute function we make the kernel
> RW since we are turning the MMO off and nuking the old kernel anyway.
>
>> Will
>
> Regards,
> Nikolay
>
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-- 
Kees Cook
Chrome OS Security