[RFC PATCH v1 12/28] riscv mm: manufacture shadow stack pte

[debug at rivosinc.com]

From: Deepak Gupta <debug at rivosinc.com>

This patch implements creating shadow stack pte (on riscv). Creating
shadow stack PTE on riscv means that clearing RWX and then setting W=1.

Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 arch/riscv/include/asm/pgtable.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 54a8dde29504..7ed00b4cc73d 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -408,6 +408,12 @@ static inline pte_t pte_mkwrite_novma(pte_t pte)
 	return __pte(pte_val(pte) | _PAGE_WRITE);
 }
 
+static inline pte_t pte_mkwrite_shstk(pte_t pte)
+{
+	/* shadow stack on risc-v is XWR = 010. Clear everything and only set _PAGE_WRITE */
+	return __pte((pte_val(pte) & ~(_PAGE_LEAF)) | _PAGE_WRITE);
+}
+
 /* static inline pte_t pte_mkexec(pte_t pte) */
 
 static inline pte_t pte_mkdirty(pte_t pte)
@@ -705,6 +711,12 @@ static inline pmd_t pmd_mkwrite_novma(pmd_t pmd)
 	return pte_pmd(pte_mkwrite_novma(pmd_pte(pmd)));
 }
 
+static inline pmd_t pmd_mkwrite_shstk(pmd_t pte)
+{
+	/* shadow stack on risc-v is XWR = 010. Clear everything and only set _PAGE_WRITE */
+	return __pmd((pmd_val(pte) & ~(_PAGE_LEAF)) | _PAGE_WRITE);
+}
+
 static inline pmd_t pmd_wrprotect(pmd_t pmd)
 {
 	return pte_pmd(pte_wrprotect(pmd_pte(pmd)));
-- 
2.43.0