linux-kernel - Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs

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Date:   Tue, 14 Feb 2023 13:10:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Deepak Gupta <debug@...osinc.com>
Cc:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to
 manufacture shadow stack PTEs

On 13.02.23 21:01, Deepak Gupta wrote:
> On Mon, Feb 13, 2023 at 03:56:22PM +0100, David Hildenbrand wrote:
>> On 13.02.23 15:37, Deepak Gupta wrote:
>>> On Mon, Feb 13, 2023 at 01:05:16PM +0100, David Hildenbrand wrote:
>>>> On 13.02.23 05:53, Deepak Gupta wrote:
>>>>> maybe_mkwrite creates PTEs with WRITE encodings for underlying arch if
>>>>> VM_WRITE is turned on in vma->vm_flags. Shadow stack memory is a write-
>>>>> able memory except it can only be written by certain specific
>>>>> instructions. This patch allows maybe_mkwrite to create shadow stack PTEs
>>>>> if vma is shadow stack VMA. Each arch can define which combination of VMA
>>>>> flags means a shadow stack.
>>>>>
>>>>> Additionally pte_mkshdwstk must be provided by arch specific PTE
>>>>> construction headers to create shadow stack PTEs. (in arch specific
>>>>> pgtable.h).
>>>>>
>>>>> This patch provides dummy/stub pte_mkshdwstk if CONFIG_USER_SHADOW_STACK
>>>>> is not selected.
>>>>>
>>>>> Signed-off-by: Deepak Gupta <debug@...osinc.com>
>>>>> ---
>>>>>   include/linux/mm.h      | 23 +++++++++++++++++++++--
>>>>>   include/linux/pgtable.h |  4 ++++
>>>>>   2 files changed, 25 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index 8f857163ac89..a7705bc49bfe 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -1093,6 +1093,21 @@ static inline unsigned long thp_size(struct page *page)
>>>>>   void free_compound_page(struct page *page);
>>>>>   #ifdef CONFIG_MMU
>>>>> +
>>>>> +#ifdef CONFIG_USER_SHADOW_STACK
>>>>> +bool arch_is_shadow_stack_vma(struct vm_area_struct *vma);
>>>>> +#endif
>>>>> +
>>>>> +static inline bool
>>>>> +is_shadow_stack_vma(struct vm_area_struct *vma)
>>>>> +{
>>>>> +#ifdef CONFIG_USER_SHADOW_STACK
>>>>> +	return arch_is_shadow_stack_vma(vma);
>>>>> +#else
>>>>> +	return false;
>>>>> +#endif
>>>>> +}
>>>>> +
>>>>>   /*
>>>>>    * Do pte_mkwrite, but only if the vma says VM_WRITE.  We do this when
>>>>>    * servicing faults for write access.  In the normal case, do always want
>>>>> @@ -1101,8 +1116,12 @@ void free_compound_page(struct page *page);
>>>>>    */
>>>>>   static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>>>>>   {
>>>>> -	if (likely(vma->vm_flags & VM_WRITE))
>>>>> -		pte = pte_mkwrite(pte);
>>>>> +	if (likely(vma->vm_flags & VM_WRITE)) {
>>>>> +		if (unlikely(is_shadow_stack_vma(vma)))
>>>>> +			pte = pte_mkshdwstk(pte);
>>>>> +		else
>>>>> +			pte = pte_mkwrite(pte);
>>>>> +	}
>>>>>   	return pte;
>>>>
>>>> Exactly what we are trying to avoid in the x86 approach right now.
>>>> Please see the x86 series on details, we shouldn't try reinventing the
>>>> wheel but finding a core-mm approach that fits multiple architectures.
>>>>
>>>> https://lkml.kernel.org/r/20230119212317.8324-1-rick.p.edgecombe@intel.com
>>>
>>> Thanks David for comment here. I looked at x86 approach. This patch
>>> actually written in a way which is not re-inventing wheel and is following
>>> a core-mm approach that fits multiple architectures.
>>>
>>> Change above checks `is_shadow_stack_vma` and if it returns true then only
>>> it manufactures shadow stack pte else it'll make a regular writeable mapping.
>>>
>>> Now if we look at `is_shadow_stack_vma` implementation, it returns false if
>>> `CONFIG_USER_SHADOW_STACK` is not defined. If `CONFIG_USER_SHADOW_STACK is
>>> defined then it calls `arch_is_shadow_stack_vma` which should be implemented
>>> by arch specific code. This allows each architecture to define their own vma
>>> flag encodings for shadow stack (riscv chooses presence of only `VM_WRITE`
>>> which is analogous to choosen PTE encodings on riscv W=1,R=0,X=0)
>>>
>>> Additionally pte_mkshdwstk will be nop if not implemented by architecture.
>>>
>>> Let me know if this make sense. If I am missing something here, let me know.
>>
>> See the discussion in that thread. The idea is to pass a VMA to
>> pte_mkwrite() and let it handle how to actually set it writable.
>>
> 
> Thanks. I see. Instances where `pte_mkwrite` is directly invoked by checking
> VM_WRITE and thus instead of fixing all those instance, make pte_mkwrite itself
> take vma flag or vma.
> 
> I'll revise.

Thanks, it would be great to discuss in the other threads what else you 
would need to make it work for you. I assume Rick will have something to 
play with soonish (Right, Rick? :) ).

-- 
Thanks,

David / dhildenb