Re: [PATCH v11 10/27] riscv/mm: Implement map_shadow_stack() syscall

[Zong Li]

On Mon, Mar 10, 2025 at 11:42 PM Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
>
> As discussed extensively in the changelog for the addition of this
> syscall on x86 ("x86/shstk: Introduce map_shadow_stack syscall") the
> existing mmap() and madvise() syscalls do not map entirely well onto the
> security requirements for shadow stack memory since they lead to windows
> where memory is allocated but not yet protected or stacks which are not
> properly and safely initialised. Instead a new syscall map_shadow_stack()
> has been defined which allocates and initialises a shadow stack page.
>
> This patch implements this syscall for riscv. riscv doesn't require token
> to be setup by kernel because user mode can do that by itself. However to
> provide compatibility and portability with other architectues, user mode
> can specify token set flag.
>
> Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> ---
>  arch/riscv/kernel/Makefile  |   1 +
>  arch/riscv/kernel/usercfi.c | 144 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 145 insertions(+)
>
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index 8d186bfced45..3a861d320654 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -125,3 +125,4 @@ obj-$(CONFIG_ACPI)          += acpi.o
>  obj-$(CONFIG_ACPI_NUMA)        += acpi_numa.o
>
>  obj-$(CONFIG_GENERIC_CPU_VULNERABILITIES) += bugs.o
> +obj-$(CONFIG_RISCV_USER_CFI) += usercfi.o
> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> new file mode 100644
> index 000000000000..24022809a7b5
> --- /dev/null
> +++ b/arch/riscv/kernel/usercfi.c
> @@ -0,0 +1,144 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2024 Rivos, Inc.
> + * Deepak Gupta <debug@xxxxxxxxxxxx>
> + */
> +
> +#include <linux/sched.h>
> +#include <linux/bitops.h>
> +#include <linux/types.h>
> +#include <linux/mm.h>
> +#include <linux/mman.h>
> +#include <linux/uaccess.h>
> +#include <linux/sizes.h>
> +#include <linux/user.h>
> +#include <linux/syscalls.h>
> +#include <linux/prctl.h>
> +#include <asm/csr.h>
> +#include <asm/usercfi.h>
> +
> +#define SHSTK_ENTRY_SIZE sizeof(void *)
> +
> +/*
> + * Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen
> + * implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to
> + * shadow stack. To keep it simple, we plan to use `ssamoswap` to perform writes on shadow
> + * stack.
> + */
> +static noinline unsigned long amo_user_shstk(unsigned long *addr, unsigned long val)
> +{
> +       /*
> +        * Never expect -1 on shadow stack. Expect return addresses and zero
> +        */
> +       unsigned long swap = -1;
> +
> +       __enable_user_access();
> +       asm goto(
> +               ".option push\n"
> +               ".option arch, +zicfiss\n"
> +               "1: ssamoswap.d %[swap], %[val], %[addr]\n"
> +               _ASM_EXTABLE(1b, %l[fault])
> +               RISCV_ACQUIRE_BARRIER
> +               ".option pop\n"
> +               : [swap] "=r" (swap), [addr] "+A" (*addr)
> +               : [val] "r" (val)
> +               : "memory"
> +               : fault
> +               );
> +       __disable_user_access();
> +       return swap;
> +fault:
> +       __disable_user_access();
> +       return -1;
> +}
> +
> +/*
> + * Create a restore token on the shadow stack.  A token is always XLEN wide
> + * and aligned to XLEN.
> + */
> +static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
> +{
> +       unsigned long addr;
> +
> +       /* Token must be aligned */
> +       if (!IS_ALIGNED(ssp, SHSTK_ENTRY_SIZE))
> +               return -EINVAL;
> +
> +       /* On RISC-V we're constructing token to be function of address itself */
> +       addr = ssp - SHSTK_ENTRY_SIZE;
> +
> +       if (amo_user_shstk((unsigned long __user *)addr, (unsigned long)ssp) == -1)
> +               return -EFAULT;
> +
> +       if (token_addr)
> +               *token_addr = addr;
> +
> +       return 0;
> +}
> +
> +static unsigned long allocate_shadow_stack(unsigned long addr, unsigned long size,
> +                                          unsigned long token_offset, bool set_tok)
> +{
> +       int flags = MAP_ANONYMOUS | MAP_PRIVATE;
> +       struct mm_struct *mm = current->mm;
> +       unsigned long populate, tok_loc = 0;
> +
> +       if (addr)
> +               flags |= MAP_FIXED_NOREPLACE;
> +
> +       mmap_write_lock(mm);
> +       addr = do_mmap(NULL, addr, size, PROT_READ, flags,
> +                      VM_SHADOW_STACK | VM_WRITE, 0, &populate, NULL);
> +       mmap_write_unlock(mm);
> +
> +       if (!set_tok || IS_ERR_VALUE(addr))
> +               goto out;
> +
> +       if (create_rstor_token(addr + token_offset, &tok_loc)) {
> +               vm_munmap(addr, size);
> +               return -EINVAL;
> +       }
> +
> +       addr = tok_loc;
> +
> +out:
> +       return addr;
> +}
> +
> +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
> +{
> +       bool set_tok = flags & SHADOW_STACK_SET_TOKEN;
> +       unsigned long aligned_size = 0;
> +
> +       if (!cpu_supports_shadow_stack())
> +               return -EOPNOTSUPP;
> +
> +       /* Anything other than set token should result in invalid param */
> +       if (flags & ~SHADOW_STACK_SET_TOKEN)
> +               return -EINVAL;
> +
> +       /*
> +        * Unlike other architectures, on RISC-V, SSP pointer is held in CSR_SSP and is available
> +        * CSR in all modes. CSR accesses are performed using 12bit index programmed in instruction
> +        * itself. This provides static property on register programming and writes to CSR can't
> +        * be unintentional from programmer's perspective. As long as programmer has guarded areas
> +        * which perform writes to CSR_SSP properly, shadow stack pivoting is not possible. Since
> +        * CSR_SSP is writeable by user mode, it itself can setup a shadow stack token subsequent
> +        * to allocation. Although in order to provide portablity with other architecture (because
> +        * `map_shadow_stack` is arch agnostic syscall), RISC-V will follow expectation of a token
> +        * flag in flags and if provided in flags, setup a token at the base.
> +        */
> +
> +       /* If there isn't space for a token */
> +       if (set_tok && size < SHSTK_ENTRY_SIZE)
> +               return -ENOSPC;
> +
> +       if (addr && (addr & (PAGE_SIZE - 1)))
> +               return -EINVAL;
> +
> +       aligned_size = PAGE_ALIGN(size);
> +       if (aligned_size < size)
> +               return -EOVERFLOW;
> +
> +       return allocate_shadow_stack(addr, aligned_size, size, set_tok);
> +}
>
LGTM.

Reviewed-by: Zong Li <zong.li@xxxxxxxxxx>
> --
> 2.34.1
>
>
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