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

David Hildenbrand
2023-02-14T12:10:25Z

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@rivosinc.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