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Date: Thu, 21 Jun 2018 19:27:01 +0200
From: Ard Biesheuvel <>
To: James Morse <>
Cc: Jun Yao <>, 
	linux-arm-kernel <>, 
	Catalin Marinas <>, Will Deacon <>, 
	Linux Kernel Mailing List <>, 
	Kernel Hardening <>
Subject: Re: [PATCH 1/1] arm64/mm: move {idmap_pg_dir,tramp_pg_dir,swapper_pg_dir}
 to .rodata section

On 21 June 2018 at 19:04, James Morse <> wrote:
> Hi Ard,
> On 21/06/18 10:29, Ard Biesheuvel wrote:
>> On 21 June 2018 at 10:59, James Morse <> wrote:
>>> On 21/06/18 07:39, Ard Biesheuvel wrote:
>>>> On 21 June 2018 at 04:51, Jun Yao <> wrote:
>>>>> On Wed, Jun 20, 2018 at 12:09:49PM +0200, Ard Biesheuvel wrote:
>>>>>> On 20 June 2018 at 10:57, Jun Yao <> 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)
>> It already is.
> Sorry, I missed that,
>>>>>> 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.
>> The attack, you mean? Because this code is unlikely to build with that
>> configuration, given that __pgd_populate() BUILD_BUG()s in that case.
> Yes, the attack. (I struggle to think of it as an 'attack' because you already
> have arbitrary write...)

OK, so in that case, you can abuse your single arbitrary write to map
an entire 1 GB block of memory with arbitrary permissions, allowing
userland to take control of the contents, right? And if you know the
virtual and physical addresses of swapper_pg_dir, you can make sure
this block covers the entire kernel, allowing the attacker to
manipulate all core kernel code and statically allocated data

What I don't understand about this patch is how it is sufficient to
only remap swapper_pg_dir r/w for updates on kernels that use 4 level

>>>>>> @@ -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().
>> Is that complexity truly justified for a security sensitive piece of
>> code?
> Wouldn't this be less complex? (I've probably explained it badly.)
> Today head.S builds the initial page tables in ~3 levels of swapper_pg_dir, then
> during paging_init() build new tables with a temporary top level.
> We switch to the temporary top level, then copy over the first level of
> swapper_pg_dir, then switch back to swapper_pg_dir. Finally we free the
> no-longer-used levels of swapper_pg_dir.
> This looks like re-inventing __initdata for the bits of page table we eventually
> free.
> What I tried to describe is building the head.S/initial-page-tables in a
> reserved area of the the __initdata section. We no longer need a temporary
> top-level, we can build the final page tables directly in swapper_pg_dir, which
> means one fewer rounds of cpu_replace_ttbr1().

Ah fair enough. So either the initial page tables are never referred
to via swapper_pg_dir in the first place, or we copy the first level
over from __initdata after setting it up (which is probably easier
than teaching the asm code about non-consecutive page ranges).

So indeed, that would be an improvement in its own right.

>> Can't we just drop the memblock_free() and be done with it?
> That works, I assumed it would be at least frowned on!

I think I prefer your suggestion above.

But we do need to teach this code to deal with folded page table levels.

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