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

[Boris Ostrovsky]

On 12/19/2016 08:27 PM, Boris Ostrovsky wrote:
> 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]
> >            ^^^^^^^^^^^^^^


Sorry, forgot about this: I see the first line but not the other two (so 
the relocation did not occur).


-boris

> > 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.