Re: [PATCH v11 06/27] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

[Zong Li <zong.li@sifive.com>]

On Mon, Mar 10, 2025 at 11:42 PM Deepak Gupta <debug@rivosinc.com> 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    | 25 +++++++++++++++++++++++++
>  arch/riscv/include/asm/pgtable.h |  1 +
>  arch/riscv/kernel/sys_riscv.c    | 10 ++++++++++
>  arch/riscv/mm/init.c             |  2 +-
>  4 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h
> new file mode 100644
> index 000000000000..392c9c2d2e78
> --- /dev/null
> +++ b/arch/riscv/include/asm/mman.h
> @@ -0,0 +1,25 @@
> +/* 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 050fdc49b5ad..8c528cd7347a 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -178,6 +178,7 @@ extern struct pt_alloc_ops pt_ops __meminitdata;
>  #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 d77afe05578f..43a448bf254b 100644
> --- a/arch/riscv/kernel/sys_riscv.c
> +++ b/arch/riscv/kernel/sys_riscv.c
> @@ -7,6 +7,7 @@
>
>  #include <linux/syscalls.h>
>  #include <asm/cacheflush.h>
> +#include <asm-generic/mman-common.h>
>
>  static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>                            unsigned long prot, unsigned long flags,
> @@ -16,6 +17,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>         if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>                 return -EINVAL;
>
> +       /*
> +        * If 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 15b2eda4c364..9d6661638d0b 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -342,7 +342,7 @@ 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,
>

LGTM.

Reviewed-by: Zong Li <zong.li@sifive.com>

> --
> 2.34.1
>
>
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