Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

[H. Peter Anvin]

I don't see why we can't randomize anywhere in physical space.  We already handle the kernel above 4 GB and it wouldn't be hard to do the equivalent for the decompress/relocation code, using a #PF handler.  Not all CPUs support 1 GB pages.

On October 14, 2014 8:37:01 PM PDT, Baoquan He <bhe@xxxxxxxxxx> wrote:
>On 10/14/14 at 08:49am, Vivek Goyal wrote:
>> On Mon, Oct 13, 2014 at 01:22:42PM -0400, Vivek Goyal wrote:
>> > On Mon, Oct 13, 2014 at 08:43:00AM -0700, H. Peter Anvin wrote:
>> > > On 10/13/2014 08:19 AM, Vivek Goyal wrote:
>> > > >>>
>> > > >>> This really shouldn't have happened this way on x86-64.  It
>has to happen
>> > > >>> this way on i386, but I worry that this may be a serious
>misdesign in kaslr
>> > > >>> on x86-64.  I'm also wondering if there is any other fallout
>of this?
>> > > >>
>> > > >> I agree. On x86_64, we should stick to previous design and
>this new
>> > > >> logic of performing relocations does not sound very clean and
>makes
>> > > >> things very confusing.
>> > > >>
>> > > >> I am wondering that why couldn't we simply adjust page tables
>in case of
>> > > >> kaslr on x86_64, instead of performing relocations.
>> > > > 
>> > > > Well, IIUC, if virtual addresses are shifted w.r.t what virtual
>address
>> > > > kernel was compiled for, then relocation will have to be done.
>> > > > 
>> > > > So question will be if physical address shift is enough for
>kaslr or
>> > > > virtual address shift is necessary.
>> > > > 
>> > > 
>> > > I would assume that without a virtual address shift kaslr is
>pretty darn
>> > > pointless.  Without the physical address shift the 1:1 map can be
>used,
>> > > and again, kaslr becomes pointless.  However, there is absolutely
>no
>> > > reason why they should be coupled.  They can, in fact, be
>independently
>> > > randomized.
>> > 
>> > Agreed. On x86_64, we should be able to randomize virtual address
>space
>> > and physical address space independently. And in that case whole of
>> > the physical memory should be available for a possible location for
>> > kernel. (As opposed to a small limit (I guess 1GB) now)
>
>It can be done to randomize virtual address space and physical address
>space independently. But limited by the 2G of kernel text mapping and
>module mapping virtual address space, virtual address can be randomized
>in (0x1000000, 1G) range. While physical address can be randomized in
>(0x1000000, 4G) according to the identity mapping of normal kernel.
>Then
>phys_base still stores an relative value, a different offset than
>before.
>
>This can be easily implement. One thing is still there's a limit for
>physical addr randomization, only below 4G. So I am wondering if we can
>extend the identify mapping to complete mapping of 48 bit, using 1G
>page
>frame. This can make physical addr be randomized to anywhere.
>
>So now there may be 3 options:
>
>1) Fix this bug in current kaslr. Since when randomize the new kernel
>location in choose_kernel_location(), cmdline options has been checked
>strictly, e.g if nokaslr is specified, it's safe to do the kernel
>location randomization. Then in handle_relocations(), we only need to
>check if the kernel location is changed, comparing with kernel loaded
>addr. If changed, kaslr is done, let's do the relocation handling. If
>not changed, no kaslr id done, just skip the relocation handling like
>before.
>
>2) randomize the virtual addr space and physical addr space
>independently. But physical addr space must be below 4G.
>
>3) extend the identity mapping to 48bit of addr space. Then we can
>randomized the virtual addr space in (0x1000000, 1G) and physical addr
>space in (0x1000000, real physical memory end).
>
>If option 3 is doable, it's the best. If not, I think bug fix should be
>better.
>
>> 
>> Hi Peter,
>> 
>> So what do we do about this issue in short term to make kexec work.
>Even
>> if we go for above solution, to make kexec work we will have to pass
>> "nokaslr" as we don't want kernel to move around in physical address
>space
>> as it might stomp over ELF headers we have stored.
>
>kexec doesn't need ELF headers. Kdump may need it. But in current
>kexec-tools implementation, kernel/initrd and other stuffs are placed
>from top to down, current implementation won't do kaslr since it only
>happened between kernel loaded addr and 1G. So we don't need to worry
>about the stomping.
>
>> 
>> If you don't like current patch, should we just disable relocations
>in
>> x86_64 if "nokaslr" command line is passed. That way kernel will not
>> be moved in physical as well as virtual address space.
>> 
>> Thanks
>> Vivek

-- 
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