[RFC,v1,15/28] riscv/mm: Implement map_shadow_stack() syscall

Commit Message

Deepak Gupta Jan. 25, 2024, 6:21 a.m. UTC

From: Deepak Gupta <[e-mail]>

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 guarded control stacks 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 compatiblity and portability with other architectues, user mode can
specify token set flag.

Signed-off-by: Deepak Gupta <[e-mail]>
---
 arch/riscv/kernel/Makefile      |   2 +
 arch/riscv/kernel/usercfi.c     | 150 ++++++++++++++++++++++++++++++++
 include/uapi/asm-generic/mman.h |   1 +
 3 files changed, 153 insertions(+)
 create mode 100644 arch/riscv/kernel/usercfi.c

Comments

Charlie Jenkins Jan. 25, 2024, 9:24 p.m. UTC | #1

On Wed, Jan 24, 2024 at 10:21:40PM -0800, [e-mail] wrote:
> From: Deepak Gupta <[e-mail]>
> 
> 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 guarded control stacks 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 compatiblity and portability with other architectues, user mode can
> specify token set flag.
> 
> Signed-off-by: Deepak Gupta <[e-mail]>
> ---
>  arch/riscv/kernel/Makefile      |   2 +
>  arch/riscv/kernel/usercfi.c     | 150 ++++++++++++++++++++++++++++++++
>  include/uapi/asm-generic/mman.h |   1 +
>  3 files changed, 153 insertions(+)
>  create mode 100644 arch/riscv/kernel/usercfi.c
> 
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index fee22a3d1b53..8c668269e886 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -102,3 +102,5 @@ obj-$(CONFIG_COMPAT)		+= compat_vdso/
>  
>  obj-$(CONFIG_64BIT)		+= pi/
>  obj-$(CONFIG_ACPI)		+= acpi.o
> +
> +obj-$(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..35ede2cbc05b
> --- /dev/null
> +++ b/arch/riscv/kernel/usercfi.c
> @@ -0,0 +1,150 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2023 Rivos, Inc.
Nit: Should be updated to 2024
> + * Deepak Gupta <[e-mail]>
> + */
> +
> +#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)
> +{
> +	/*
> +	 * In case ssamoswap faults, return -1.
> +	 * Never expect -1 on shadow stack. Expect return addresses and zero
> +	 */
> +	unsigned long swap = -1;
> +
> +	__enable_user_access();
> +	asm_volatile_goto(
> +				".option push\n"
> +				".option arch, +zicfiss\n"
> +#ifdef CONFIG_64BIT
> +				"1: ssamoswap.d %0, %2, %1\n"
> +#else
> +				"1: ssamoswap.w %0, %2, %1\n"

A SSAMOSWAP macro that conditionally defines this would be cleaner

> +#endif
> +				_ASM_EXTABLE(1b, %l[fault])
> +				RISCV_ACQUIRE_BARRIER
> +				".option pop\n"
> +				: "=r" (swap), "+A" (*addr)

I just ran into this on one of my patches that not every compiler
supports output args in asm goto blocks. You need to guard this with the
kconfig option CC_HAS_ASM_GOTO_TIED_OUTPUT. Unfortunately, that means
that this code needs two versions, or you can choose to gate CFI behind
this option, it's supported by recent versions of GCC/CLANG.

For readability it is also nice to use labels for the asm variables such
as `"=r" (swap)` can be `[swap] "=r" (swap)` and then replace %0 with
%[swap].

- Charlie

> +				: "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_SHADOWSTACK, flags,
> +				VM_SHADOW_STACK, 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))
> +		return -EINVAL;
> +
> +	aligned_size = PAGE_ALIGN(size);
> +	if (aligned_size < size)
> +		return -EOVERFLOW;
> +
> +	return allocate_shadow_stack(addr, aligned_size, size, set_tok);
> +}
> diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
> index 57e8195d0b53..0c0ac6214de6 100644
> --- a/include/uapi/asm-generic/mman.h
> +++ b/include/uapi/asm-generic/mman.h
> @@ -19,4 +19,5 @@
>  #define MCL_FUTURE	2		/* lock all future mappings */
>  #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
>  
> +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
>  #endif /* __ASM_GENERIC_MMAN_H */
> -- 
> 2.43.0
>

Deepak Gupta Jan. 26, 2024, 12:44 a.m. UTC | #2

On Thu, Jan 25, 2024 at 01:24:16PM -0800, Charlie Jenkins wrote:
>On Wed, Jan 24, 2024 at 10:21:40PM -0800, [e-mail] wrote:
>> From: Deepak Gupta <[e-mail]>
>>
>> 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 guarded control stacks 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 compatiblity and portability with other architectues, user mode can
>> specify token set flag.
>>
>> Signed-off-by: Deepak Gupta <[e-mail]>
>> ---
>>  arch/riscv/kernel/Makefile      |   2 +
>>  arch/riscv/kernel/usercfi.c     | 150 ++++++++++++++++++++++++++++++++
>>  include/uapi/asm-generic/mman.h |   1 +
>>  3 files changed, 153 insertions(+)
>>  create mode 100644 arch/riscv/kernel/usercfi.c
>>
>> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
>> index fee22a3d1b53..8c668269e886 100644
>> --- a/arch/riscv/kernel/Makefile
>> +++ b/arch/riscv/kernel/Makefile
>> @@ -102,3 +102,5 @@ obj-$(CONFIG_COMPAT)		+= compat_vdso/
>>
>>  obj-$(CONFIG_64BIT)		+= pi/
>>  obj-$(CONFIG_ACPI)		+= acpi.o
>> +
>> +obj-$(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..35ede2cbc05b
>> --- /dev/null
>> +++ b/arch/riscv/kernel/usercfi.c
>> @@ -0,0 +1,150 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2023 Rivos, Inc.
>Nit: Should be updated to 2024

noted

>> + * Deepak Gupta <[e-mail]>
>> + */
>> +
>> +#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)
>> +{
>> +	/*
>> +	 * In case ssamoswap faults, return -1.
>> +	 * Never expect -1 on shadow stack. Expect return addresses and zero
>> +	 */
>> +	unsigned long swap = -1;
>> +
>> +	__enable_user_access();
>> +	asm_volatile_goto(
>> +				".option push\n"
>> +				".option arch, +zicfiss\n"
>> +#ifdef CONFIG_64BIT
>> +				"1: ssamoswap.d %0, %2, %1\n"
>> +#else
>> +				"1: ssamoswap.w %0, %2, %1\n"
>
>A SSAMOSWAP macro that conditionally defines this would be cleaner

Yes I need to do that. Infact I need to gate CONFIG_RISCV_USER_CFI behind
some riscv-gnu toolchain version as well. Becuase not all toolchain versions will
recognize this.

>
>> +#endif
>> +				_ASM_EXTABLE(1b, %l[fault])
>> +				RISCV_ACQUIRE_BARRIER
>> +				".option pop\n"
>> +				: "=r" (swap), "+A" (*addr)
>
>I just ran into this on one of my patches that not every compiler
>supports output args in asm goto blocks. You need to guard this with the
>kconfig option CC_HAS_ASM_GOTO_TIED_OUTPUT. Unfortunately, that means
>that this code needs two versions, or you can choose to gate CFI behind
>this option, it's supported by recent versions of GCC/CLANG.

Thanks.
I'll gate behind CC_HAS_ASM_GOTO_TIED_OUTPUT. Earlier versions of GCC/CLANG
won't have CFI support in them anyways.

>
>For readability it is also nice to use labels for the asm variables such
>as `"=r" (swap)` can be `[swap] "=r" (swap)` and then replace %0 with
>%[swap].

noted, will do that. I copied it from gcc asm snippet `amoswap` somewhere in
kernel. Goes without saying, I am terrible with gcc asm syntax.

>
>- Charlie
>
>> +				: "r" (val)
>> +				: "memory"
>> +				: fault

Mark Brown Feb. 6, 2024, 4:01 p.m. UTC | #3

On Wed, Jan 24, 2024 at 10:21:40PM -0800, [e-mail] 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 guarded control stacks 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.

While I agree that this is very well written you probably want to update
the references to guarded control stacks to whatever the RISC-V term is :P

> --- a/include/uapi/asm-generic/mman.h
> +++ b/include/uapi/asm-generic/mman.h
> @@ -19,4 +19,5 @@
>  #define MCL_FUTURE	2		/* lock all future mappings */
>  #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
>  
> +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
>  #endif /* __ASM_GENERIC_MMAN_H */

For arm64 I also added a SHADOW_STACK_SET_MARKER for adding a top of
stack marker, did you have any thoughts on that for RISC-V?  I think x86
were considering adding it too, it'd be good if we could get things
consistent.

Edgecombe, Rick P Feb. 9, 2024, 8:44 p.m. UTC | #4

On Wed, 2024-01-24 at 22:21 -0800, [e-mail] wrote:
> From: Deepak Gupta <[e-mail]>
> 
> 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 guarded control stacks 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 compatiblity and portability with other architectues, user
> mode can
> specify token set flag.

A lot of this code look very familiar. We'll have to think about at
what point we could pull some of it into the code kernel.

I think if we had an arch write_user_shstk(), most of the code could be
shared here.

Deepak Gupta Feb. 22, 2024, 12:47 a.m. UTC | #5

On Tue, Feb 06, 2024 at 04:01:28PM +0000, Mark Brown wrote:
>On Wed, Jan 24, 2024 at 10:21:40PM -0800, [e-mail] 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 guarded control stacks 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.
>
>While I agree that this is very well written you probably want to update
>the references to guarded control stacks to whatever the RISC-V term is :P

Noted. I'll do that in next patchset.

>
>> --- a/include/uapi/asm-generic/mman.h
>> +++ b/include/uapi/asm-generic/mman.h
>> @@ -19,4 +19,5 @@
>>  #define MCL_FUTURE	2		/* lock all future mappings */
>>  #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
>>
>> +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
>>  #endif /* __ASM_GENERIC_MMAN_H */
>
>For arm64 I also added a SHADOW_STACK_SET_MARKER for adding a top of
>stack marker, did you have any thoughts on that for RISC-V?  I think x86
>were considering adding it too, it'd be good if we could get things
>consistent.

Please correct me on this. A token at the top which can't be consumed to restore
but *just* purely as marker, right?
It's a good design basic with not a lot of cost.

I think risc-v should be able to converge on that.

Deepak Gupta Feb. 22, 2024, 12:50 a.m. UTC | #6

On Fri, Feb 09, 2024 at 08:44:53PM +0000, Edgecombe, Rick P wrote:
>On Wed, 2024-01-24 at 22:21 -0800, [e-mail] wrote:
>> From: Deepak Gupta <[e-mail]>
>>
>> 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 guarded control stacks 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 compatiblity and portability with other architectues, user
>> mode can
>> specify token set flag.
>
>A lot of this code look very familiar. We'll have to think about at
>what point we could pull some of it into the code kernel.
>
>I think if we had an arch write_user_shstk(), most of the code could be
>shared here.

Yes it is. I'll think a little bit more on this on next set of patchsets
when I send.

Mark Brown Feb. 22, 2024, 1:33 p.m. UTC | #7

On Wed, Feb 21, 2024 at 04:47:11PM -0800, Deepak Gupta wrote:
> On Tue, Feb 06, 2024 at 04:01:28PM +0000, Mark Brown wrote:

> > > +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */

> > For arm64 I also added a SHADOW_STACK_SET_MARKER for adding a top of
> > stack marker, did you have any thoughts on that for RISC-V?  I think x86
> > were considering adding it too, it'd be good if we could get things
> > consistent.

> Please correct me on this. A token at the top which can't be consumed to restore
> but *just* purely as marker, right?

Yes, for arm64 we just leave a zero word (which can't be a valid token)
above the stack switch token, that does mean you can't exactly tell that
the top of stack marker is there unless there's also a stack switch
token below it.

> It's a good design basic with not a lot of cost.

> I think risc-v should be able to converge on that.

Great.

Patch

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index fee22a3d1b53..8c668269e886 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -102,3 +102,5 @@  obj-$(CONFIG_COMPAT)		+= compat_vdso/
 
 obj-$(CONFIG_64BIT)		+= pi/
 obj-$(CONFIG_ACPI)		+= acpi.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..35ede2cbc05b
--- /dev/null
+++ b/arch/riscv/kernel/usercfi.c
@@ -0,0 +1,150 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2023 Rivos, Inc.
+ * Deepak Gupta <[e-mail]>
+ */
+
+#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)
+{
+	/*
+	 * In case ssamoswap faults, return -1.
+	 * Never expect -1 on shadow stack. Expect return addresses and zero
+	 */
+	unsigned long swap = -1;
+
+	__enable_user_access();
+	asm_volatile_goto(
+				".option push\n"
+				".option arch, +zicfiss\n"
+#ifdef CONFIG_64BIT
+				"1: ssamoswap.d %0, %2, %1\n"
+#else
+				"1: ssamoswap.w %0, %2, %1\n"
+#endif
+				_ASM_EXTABLE(1b, %l[fault])
+				RISCV_ACQUIRE_BARRIER
+				".option pop\n"
+				: "=r" (swap), "+A" (*addr)
+				: "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_SHADOWSTACK, flags,
+				VM_SHADOW_STACK, 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))
+		return -EINVAL;
+
+	aligned_size = PAGE_ALIGN(size);
+	if (aligned_size < size)
+		return -EOVERFLOW;
+
+	return allocate_shadow_stack(addr, aligned_size, size, set_tok);
+}
diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
index 57e8195d0b53..0c0ac6214de6 100644
--- a/include/uapi/asm-generic/mman.h
+++ b/include/uapi/asm-generic/mman.h
@@ -19,4 +19,5 @@ 
 #define MCL_FUTURE	2		/* lock all future mappings */
 #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
 
+#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
 #endif /* __ASM_GENERIC_MMAN_H */