[v12,10/28] riscv/mm: Implement map_shadow_stack() syscall

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.

Reviewed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/kernel/Makefile  |   1 +
 arch/riscv/kernel/usercfi.c | 144 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 145 insertions(+)

On Sat, Mar 15, 2025 at 5:39 AM Deepak Gupta <debug@rivosinc.com> 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.
>
> Reviewed-by: Zong Li <zong.li@sifive.com>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
>  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@rivosinc.com>
> + */
> +
> +#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"

Hi Deepak,
It just came to my mind, do we need to ensure that menvcfg.SSE is not
zero before executing the ssamoswap instruction? Since ssamoswap is
not encoded using MOP, I’m wondering if we should make sure that
executing ssamoswap won’t accidentally trigger an illegal instruction
exception. Thanks.

> +               _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);
> +}
>
> --
> 2.34.1
>

On Mon, Apr 07, 2025 at 12:50:35PM +0800, Zong Li wrote:
>On Sat, Mar 15, 2025 at 5:39 AM Deepak Gupta <debug@rivosinc.com> 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.
>>
>> Reviewed-by: Zong Li <zong.li@sifive.com>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  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@rivosinc.com>
>> + */
>> +
>> +#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"
>
>Hi Deepak,
>It just came to my mind, do we need to ensure that menvcfg.SSE is not
>zero before executing the ssamoswap instruction? Since ssamoswap is
>not encoded using MOP, I’m wondering if we should make sure that
>executing ssamoswap won’t accidentally trigger an illegal instruction
>exception. Thanks.

FWFT patches turn SSE during early boot. There is a bug there though,
I need to check if those FWFT SBI call succeeded or not. If it failed
then itshould set a global variable indicating shadow stack can't be
turned on. And in that case this flow wouldn't be reachable. Soon I
will post v13 with these changes.

Thanks for noticing.
>
>> +               _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);
>> +}
>>
>> --
>> 2.34.1
>>

2025-03-14T14:39:29-07:00, Deepak Gupta <debug@rivosinc.com>:
> 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.

RISC-V shadow stack could use mmap() and madvise() perfectly well.
Userspace can always initialize the shadow stack properly and the shadow
stack memory is never protected from other malicious threads.

I think that the compatibility argument is reasonable.  We'd need to
modify the other syscalls to allow a write-only mapping anyway.

> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> +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"

Shouldn't compiler accept ssamoswap.d opcode even without zicfiss arch?

> +		"1: ssamoswap.d %[swap], %[val], %[addr]\n"
> +		_ASM_EXTABLE(1b, %l[fault])
> +		RISCV_ACQUIRE_BARRIER

Why is the barrier here?

> +		".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;

I think we should return 0 and -EFAULT.
We can ignore the swapped value, or return it through a pointer.

> +}
> +
> +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;

Is MAP_GROWSDOWN pointless?

> +	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,

PROT_READ implies VM_READ, so won't this select PAGE_COPY in the
protection_map instead of PAGE_SHADOWSTACK?

Wouldn't avoiding VM_READ also allow us to get rid of the ugly hack in
pte_mkwrite?  (VM_WRITE would naturally select the right XWR flags.)

> +		       VM_SHADOW_STACK | VM_WRITE, 0, &populate, NULL);
> +	mmap_write_unlock(mm);
> +
> +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
> +{
> [...]
> +	if (addr && (addr & (PAGE_SIZE - 1)))

if (!PAGE_ALIGNED(addr))