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

Deepak Gupta
2023-02-13T20:01:19Z

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,
>
>David / dhildenb
>