Messages in this thread | | | Date | Tue, 14 Feb 2023 13:10:06 +0100 | Subject | Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs | From | David Hildenbrand <> |
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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
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