linux-kernel - Re: [PATCH] x86/microcode: Adjust ramdisk address when accessing by virtual address

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Date:   Mon, 19 Dec 2016 20:27:29 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/microcode: Adjust ramdisk address when accessing by
 virtual address



On 12/19/2016 06:32 PM, Borislav Petkov wrote:
> On Mon, Dec 19, 2016 at 07:43:40PM +0100, Borislav Petkov wrote:
>> On Mon, Dec 19, 2016 at 01:12:25PM -0500, Boris Ostrovsky wrote:
>>> IIUIC find_microcode_in_initrd() is called with paging on only on Intel
>>> (which is where I observed it).
>>
>> Ah, that was an important fact. Yes, I can repro it now.
>
> Ok, questions:
>
> * does your guest relocate the ramdisk?


This is not a guest. I crashed with baremetal kernel.


>
> I.e., do you see something like this in dmesg before the splat:
>
> [    0.000000] RAMDISK: [mem 0x7f84c000-0x7ffcffff]
> [    0.000000] Allocated new RAMDISK: [mem 0x3647a000-0x36bfd9e6]
> [    0.000000] Move RAMDISK from [mem 0x7f84c000-0x7ffcf9e6] to [mem 0x3647a000-0x36bfd9e6]
> 	       ^^^^^^^^^^^^^^
>
> If not, then I know what happens.
>
> Also, does it work if you change these lines:
>
>         if (!use_pa && relocated_ramdisk)
>                 start = initrd_start;
>
> to:
>
>         if (!use_pa)
>                 start = initrd_start;

Yes, it does.

I also thought it might be better but I haven't gone through the code to 
make sure this would always work.

I can run more tests tomorrow if you want.

-boris

>
>
> Because if that works, I can actually simplify that function radically.
>
> But more tomorrow.
>
> Thanks.
>