Re: [PATCH 1/1] arm64/mm: move {idmap_pg_dir,tramp_pg_dir,swapper_pg_dir} to .rodata section

[James Morse <james.morse@arm.com>]

Hi guys,

On 21/06/18 07:39, Ard Biesheuvel wrote:
> On 21 June 2018 at 04:51, Jun Yao <yaojun8558363@gmail.com> wrote:
>> On Wed, Jun 20, 2018 at 12:09:49PM +0200, Ard Biesheuvel wrote:
>>> On 20 June 2018 at 10:57, Jun Yao <yaojun8558363@gmail.com> wrote:
>>>> Move {idmap_pg_dir,tramp_pg_dir,swapper_pg_dir} to .rodata
>>>> section. And update the swapper_pg_dir by fixmap.
>>>>
>>>
>>> I think we may be able to get away with not mapping idmap_pg_dir and
>>> tramp_pg_dir at all.
>>
>> I think we need to move tramp_pg_dir to .rodata. The attacker can write
>> a block-mapping(AP=01) to tramp_pg_dir and then he can access kernel
>> memory.

> Why does it need to be mapped at all? When do we ever access it from the code?

(We would want to make its fixmap entry read-only too)


>>> As for swapper_pg_dir, it would indeed be nice if we could keep those
>>> mappings read-only most of the time, but I'm not sure how useful this
>>> is if we apply it to the root level only.
>>
>> The purpose of it is to make 'KSMA' harder, where an single arbitrary
>> write is used to add a block mapping to the page-tables, giving the
>> attacker full access to kernel memory. That's why we just apply it to
>> the root level only. If the attacker can arbitrary write multiple times,
>> I think it's hard to defend.
>>
> 
> So the assumption is that the root level is more easy to find?
> Otherwise, I'm not sure I understand why being able to write a level 0
> entry is so harmful, given that we don't have block mappings at that
> level.

I think this thing assumes 3-level page tables with 39bit VA.


>>>> @@ -417,12 +421,22 @@ static void __init __map_memblock(pgd_t *pgdp, phys_addr_t start,
>>>>
>>>>  void __init mark_linear_text_alias_ro(void)
>>>>  {

>>>> +       size = (unsigned long)__init_begin - (unsigned long)swapper_pg_end;
>>>> +       update_mapping_prot(__pa_symbol(swapper_pg_end),
>>>> +                           (unsigned long)lm_alias(swapper_pg_end),
>>>> +                           size, PAGE_KERNEL_RO);
>>>
>>> I don't think this is necessary. Even if some pages are freed, it
>>> doesn't harm to keep a read-only alias of them here since the new
>>> owner won't access them via this mapping anyway. So we can keep
>>> .rodata as a single region.
>>
>> To be honest, I didn't think of this issue at first. I later found a
>> problem when testing the code on qemu:
> 
> OK, you're right. I missed the fact that this operates on the linear
> alias, not the kernel mapping itself.
> 
> What I don't like is that we lose the ability to use block mappings
> for the entire .rodata section this way. Isn't it possible to move
> these pgdirs to the end of the .rodata segment, perhaps by using a
> separate input section name and placing that explicitly? We could even
> simply forget about freeing those pages, given that [on 4k pages] the
> benefit of freeing 12 KB of space is likely to get lost in the
> rounding noise anyway [segments are rounded up to 64 KB in size]

I assumed that to move swapper_pg_dir into the .rodata section we would need to
break it up. Today its ~3 levels, which we setup in head.S, then do a dance in
paging_init() so that swapper_pg_dir is always the top level.

We could generate all leves of the 'init_pg_dir' in the __initdata section, then
copy only the top level into swapper_pg_dir into the rodata section during
paging_init().


Thanks,

James