[PATCH v1 RFC Zisslpcfi 06/20] riscv: Implementing "PROT_SHADOWSTACK" on riscv

[Deepak Gupta <debug@rivosinc.com>]

This patchimplements new mmap protection flag "PROT_SHADOWSTACK" on riscv

Zisslpcfi extension on riscv uses R=0, W=1, X=0 as shadow stack PTE
encoding. This encoding is reserved if Zisslpcfi is not implemented or
backward cfi is not enabled for the respective mode.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/include/asm/mman.h    | 19 +++++++++++++++++++
 arch/riscv/include/asm/pgtable.h |  1 +
 arch/riscv/kernel/sys_riscv.c    | 22 ++++++++++++++++++++++
 arch/riscv/mm/init.c             |  2 +-
 4 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/mman.h

diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h
new file mode 100644
index 000000000000..9c8499294a60
--- /dev/null
+++ b/arch/riscv/include/asm/mman.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <uapi/asm/mman.h>
+
+static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
+	unsigned long pkey __always_unused)
+{
+	unsigned long ret = 0;
+
+	ret = (prot & PROT_SHADOWSTACK)?VM_WRITE:0;
+	return ret;
+}
+#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
+
+#endif /* ! __ASM_MMAN_H__ */
diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 4eba9a98d0e3..74dbe122f2fa 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -159,6 +159,7 @@ extern struct pt_alloc_ops pt_ops __initdata;
 #define PAGE_READ_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
 #define PAGE_WRITE_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ |	\
 					 _PAGE_EXEC | _PAGE_WRITE)
+#define PAGE_SHADOWSTACK       __pgprot(_PAGE_BASE | _PAGE_WRITE)
 
 #define PAGE_COPY		PAGE_READ
 #define PAGE_COPY_EXEC		PAGE_EXEC
diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 5d3f2fbeb33c..c3cf6b94c710 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -18,6 +18,28 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
 	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
 		return -EINVAL;
 
+	/*
+	 * If only PROT_WRITE is specified then extend that to PROT_READ
+	 * protection_map[VM_WRITE] is now going to select shadow stack encodings.
+	 * So specifying PROT_WRITE actually should select protection_map [VM_WRITE | VM_READ]
+	 * If user wants to create shadow stack then they should specify PROT_SHADOWSTACK
+	 * protection
+	 */
+	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
+		prot |= PROT_READ;
+
+	/*
+	 * PROT_SHADOWSTACK is new protection flag. If specified with other like PROT_WRITE or
+	 * PROT_READ PROT_SHADOWSTACK takes precedence. We can do either of following
+	 *   - ensure no other protection flags are specified along with it and return EINVAL
+	 *   OR
+	 *   - ensure we clear other protection flags.
+	 * Choosing to follow former, if any other bit is set in prot, we return EINVAL
+	 * Other architectures can treat different combinations for PROT_SHADOWSTACK
+	 */
+	if (unlikely((prot & PROT_SHADOWSTACK) && (prot & ~PROT_SHADOWSTACK)))
+		return -EINVAL;
+
 	return ksys_mmap_pgoff(addr, len, prot, flags, fd,
 			       offset >> (PAGE_SHIFT - page_shift_offset));
 }
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 478d6763a01a..ba8138c90450 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -294,7 +294,7 @@ static pmd_t __maybe_unused early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAG
 static const pgprot_t protection_map[16] = {
 	[VM_NONE]					= PAGE_NONE,
 	[VM_READ]					= PAGE_READ,
-	[VM_WRITE]					= PAGE_COPY,
+	[VM_WRITE]					= PAGE_SHADOWSTACK,
 	[VM_WRITE | VM_READ]				= PAGE_COPY,
 	[VM_EXEC]					= PAGE_EXEC,
 	[VM_EXEC | VM_READ]				= PAGE_READ_EXEC,
-- 
2.25.1