linux-kernel - [PATCH v2 07/27] mm: Define VM_SHADOW

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Date: Thu, 28 Mar 2024 21:44:39 -0700
From: Deepak Gupta <debug@...osinc.com>
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Subject: [PATCH v2 07/27] mm: Define VM_SHADOW_STACK for RISC-V

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

To keep things simple, RISC-V follows the same.

Signed-off-by: Deepak Gupta <debug@...osinc.com>
---
 include/linux/mm.h | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f5a97dec5169..225af437ecba 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -352,7 +352,25 @@ 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_HIGH_ARCH_5
+
+static inline bool arch_is_shadow_stack(vm_flags_t vm_flags)
+{
+	return (vm_flags & VM_SHADOW_STACK);
+}
+
+#endif
+
+#ifndef VM_SHADOW_STACK
 # define VM_SHADOW_STACK	VM_NONE
 #endif
 
-- 
2.43.2