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Date: Mon, 13 May 2024 11:29:27 -0700
From: Deepak Gupta <debug@rivosinc.com>
To: Alexandre Ghiti <alex@ghiti.fr>
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Subject: Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ |
 VM_WRITE
Message-ID: <ZkJcB5u+0bZ2KsS+@debug.ba.rivosinc.com>
References: <20240403234054.2020347-1-debug@rivosinc.com>
 <20240403234054.2020347-11-debug@rivosinc.com>
 <c759444d-fe84-4a61-8448-80fb692c7904@ghiti.fr>
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In-Reply-To: <c759444d-fe84-4a61-8448-80fb692c7904@ghiti.fr>

On Sun, May 12, 2024 at 06:24:45PM +0200, Alexandre Ghiti wrote:
>Hi Deepak,
>
>On 04/04/2024 01:34, Deepak Gupta wrote:
>>`arch_calc_vm_prot_bits` is implemented on risc-v to return VM_READ |
>>VM_WRITE if PROT_WRITE is specified. Similarly `riscv_sys_mmap` is
>>updated to convert all incoming PROT_WRITE to (PROT_WRITE | PROT_READ).
>>This is to make sure that any existing apps using PROT_WRITE still work.
>>
>>Earlier `protection_map[VM_WRITE]` used to pick read-write PTE encodings.
>>Now `protection_map[VM_WRITE]` will always pick PAGE_SHADOWSTACK PTE
>>encodings for shadow stack. Above changes ensure that existing apps
>>continue to work because underneath kernel will be picking
>>`protection_map[VM_WRITE|VM_READ]` PTE encodings.
>>
>>Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>>---
>>  arch/riscv/include/asm/mman.h    | 24 ++++++++++++++++++++++++
>>  arch/riscv/include/asm/pgtable.h |  1 +
>>  arch/riscv/kernel/sys_riscv.c    | 11 +++++++++++
>>  arch/riscv/mm/init.c             |  2 +-
>>  mm/mmap.c                        |  1 +
>>  5 files changed, 38 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..ef9fedf32546
>>--- /dev/null
>>+++ b/arch/riscv/include/asm/mman.h
>>@@ -0,0 +1,24 @@
>>+/* 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;
>>+
>>+	/*
>>+	 * If PROT_WRITE was specified, force it to VM_READ | VM_WRITE.
>>+	 * Only VM_WRITE means shadow stack.
>>+	 */
>>+	if (prot & PROT_WRITE)
>>+		ret = (VM_READ | VM_WRITE);
>>+	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 6066822e7396..4d5983bc6766 100644
>>--- a/arch/riscv/include/asm/pgtable.h
>>+++ b/arch/riscv/include/asm/pgtable.h
>>@@ -184,6 +184,7 @@ 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_READ_EXEC
>>diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
>>index f1c1416a9f1e..846c36b1b3d5 100644
>>--- a/arch/riscv/kernel/sys_riscv.c
>>+++ b/arch/riscv/kernel/sys_riscv.c
>>@@ -8,6 +8,8 @@
>>  #include <linux/syscalls.h>
>>  #include <asm/cacheflush.h>
>>  #include <asm-generic/mman-common.h>
>>+#include <vdso/vsyscall.h>
>>+#include <asm/mman.h>
>>  static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>>  			   unsigned long prot, unsigned long flags,
>>@@ -17,6 +19,15 @@ 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 use `map_shadow_stack` syscall.
>>+	 */
>>+	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
>>+		prot |= PROT_READ;
>>+
>>  	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 fa34cf55037b..98e5ece4052a 100644
>>--- a/arch/riscv/mm/init.c
>>+++ b/arch/riscv/mm/init.c
>>@@ -299,7 +299,7 @@ pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>>  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,
>>diff --git a/mm/mmap.c b/mm/mmap.c
>>index d89770eaab6b..57a974f49b00 100644
>>--- a/mm/mmap.c
>>+++ b/mm/mmap.c
>>@@ -47,6 +47,7 @@
>>  #include <linux/oom.h>
>>  #include <linux/sched/mm.h>
>>  #include <linux/ksm.h>
>>+#include <linux/processor.h>
>>  #include <linux/uaccess.h>
>>  #include <asm/cacheflush.h>
>
>
>What happens if someone restricts the permission to PROT_WRITE using 
>mprotect()? I would say this is an issue since it would turn the pages 
>into shadow stack pages.

look at this patch in this patch series.
"riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE"

It implements `arch_calc_vm_prot_bits` for risc-v and enforces that incoming
PROT_WRITE is converted to VM_READ | VM_WRITE. And thus it'll become read/write
memory. This way `mprotect` can be used to convert a shadow stack page to
read/write memory but not a regular memory to shadow stack page.

>
>