Messages in this thread | | | Date | Mon, 13 Feb 2023 06:37:54 -0800 | From | Deepak Gupta <> | Subject | Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs |
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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.
> >-- >Thanks, > >David / dhildenb >
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