linux-kernel - Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory

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Date:   Tue, 24 Jan 2023 17:26:43 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>
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Subject: Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack
 memory

On 23.01.23 21:46, Edgecombe, Rick P wrote:
> On Mon, 2023-01-23 at 11:45 +0100, Florian Weimer wrote:
>> * David Hildenbrand:
>>
>>> On 19.01.23 22:23, Rick Edgecombe wrote:
>>>> The x86 Control-flow Enforcement Technology (CET) feature
>>>> includes a new
>>>> type of memory called shadow stack. This shadow stack memory has
>>>> some
>>>> unusual properties, which requires some core mm changes to
>>>> function
>>>> properly.
>>>> Shadow stack memory is writable only in very specific, controlled
>>>> ways.
>>>> However, since it is writable, the kernel treats it as such. As a
>>>> result
>>>> there remain many ways for userspace to trigger the kernel to
>>>> write to
>>>> shadow stack's via get_user_pages(, FOLL_WRITE) operations. To
>>>> make this a
>>>> little less exposed, block writable GUPs for shadow stack VMAs.
>>>> Still allow FOLL_FORCE to write through shadow stack protections,
>>>> as
>>>> it
>>>> does for read-only protections.
>>>
>>> So an app can simply modify the shadow stack itself by writing to
>>> /proc/self/mem ?
>>>
>>> Is that really intended? Looks like security hole to me at first
>>> sight, but maybe I am missing something important.
>>
>> Isn't it possible to overwrite GOT pointers using the same vector?
>> So I think it's merely reflecting the status quo.
> 
> There was some debate on this. /proc/self/mem can currently write
> through read-only memory which protects executable code. So should
> shadow stack get separate rules? Is ROP a worry when you can overwrite
> executable code?
> 

The question is, if there is reasonable debugging reason to keep it. I 
assume if a debugger would adjust the ordinary stack, it would have to 
adjust the shadow stack as well (oh my ...). So it sounds reasonable to 
have it in theory at least ... not sure when debugger would support 
that, but maybe they already do.

> The consensus seemed to lean towards not making special rules for this
> case, and there was some discussion that /proc/self/mem should maybe be
> hardened generally.

I agree with that. It's a debugging mechanism that a process can abuse 
to do nasty stuff to its memory that it maybe shouldn't be able to do ...

-- 
Thanks,

David / dhildenb