Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

Alexandre Ghiti
2024-05-12T16:26:18Z

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.