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

[Deepak Gupta <debug@rivosinc.com>]

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
>