[RFC PATCH v1 08/28] mm: Define VM_SHADOW_STACK for RISC-V

[debug at rivosinc.com]

From: Deepak Gupta <debug at rivosinc.com>

VM_SHADOW_STACK is defined by x86 as vm flag to mark a shadow stack vma.

x86 uses VM_HIGH_ARCH_5 bit but that limits shadow stack vma to 64bit only.
arm64 follows same path
https://lore.kernel.org/lkml/20231009-arm64-gcs-v6-12-78e55deaa4dd@kernel.org/#r

On RISC-V, write-only page table encodings are shadow stack pages. This patch
re-defines VM_WRITE only to be VM_SHADOW_STACK.

Next set of patches will set guard rail that no other mm flow can set VM_WRITE
only in vma except when specifically creating shadow stack.

Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 include/linux/mm.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..dfe0e8118669 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -352,7 +352,19 @@ extern unsigned int kobjsize(const void *objp);
  * for more details on the guard size.
  */
 # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
-#else
+#endif
+
+#ifdef CONFIG_RISCV_USER_CFI
+/*
+ * On RISC-V pte encodings for shadow stack is R=0, W=1, X=0 and thus RISCV
+ * choosing to use similar mechanism on vm_flags where VM_WRITE only means
+ * VM_SHADOW_STACK. RISCV as well doesn't support VM_SHADOW_STACK to be set
+ * with VM_SHARED.
+ */
+#define VM_SHADOW_STACK	VM_WRITE
+#endif
+
+#ifndef VM_SHADOW_STACK
 # define VM_SHADOW_STACK	VM_NONE
 #endif
 
-- 
2.43.0