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

[Deepak Gupta <debug@rivosinc.com>]

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;
 }
 
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 1159b25b0542..94b157218c73 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1736,4 +1736,8 @@ pgprot_t vm_get_page_prot(unsigned long vm_flags)			\
 }									\
 EXPORT_SYMBOL(vm_get_page_prot);
 
+#ifndef CONFIG_USER_SHADOW_STACK
+#define pte_mkshdwstk(pte) pte
+#endif
+
 #endif /* _LINUX_PGTABLE_H */
-- 
2.25.1