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@xxxxxxxxxxxx>
---
 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@xxxxxxxxx

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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