Re: [PATCH 1/2] x86/shstk: support via prctl

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Tue, 2026-07-14 at 10:47 -0500, Bill Roberts wrote:
> Historically, managing the user-space shadow stack state on x86 has
> been handled exclusively through the arch_prctl() interface via the
> ARCH_SHSTK_* operations. However, other architectures (such as arm64 and
> riscv) do not implement arch_prctl() and instead utilize the newer,
> arch-agnostic, prctl() interface (i.e. PR_GET_SHADOW_STACK_STATUS and
> PR_SET_SHADOW_STACK_STATUS).
> 
> To provide language runtimes, toolchains, and libc implementations with a
> consistent, cross-architecture interface for managing control-flow
> integrity, wire up the generic shadow stack prctl handlers for x86.

I think glibc doesn't handles this in arch code, right? So the point of this is
only your SE Linux patches. Or is there other cross-arch code that wants to
handle shadow stack?

BTW, for non security module people, can you explain why it needs this design?

> 
> Map the generic PR_SHADOW_STACK_ENABLE, PR_SHADOW_STACK_DISABLE, and
> PR_SHADOW_STACK_LOCK operations onto the underlying x86 internal CET helper
> routines. This allows portable userspace applications to toggle or query
> shadow stack states without relying on architecture-specific system calls,
> while maintaining backward compatibility with existing arch_prctl() calls.

Can you explain why not to include ARCH_SHSTK_UNLOCK?
https://lore.kernel.org/lkml/e1362732ba86990b7707d3f5b785358b77c5f896.camel@intel.com/

> 
> Signed-off-by: Bill Roberts <bill.roberts@arm.com>
> ---
>  arch/x86/kernel/shstk.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 0ca64900192f..ef3db94eec6a 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -18,6 +18,7 @@
>  #include <linux/sizes.h>
>  #include <linux/user.h>
>  #include <linux/syscalls.h>
> +#include <linux/prctl.h>
>  #include <asm/msr.h>
>  #include <asm/fpu/xstate.h>
>  #include <asm/fpu/types.h>
> @@ -630,3 +631,37 @@ bool shstk_is_enabled(void)
>  {
>  	return features_enabled(ARCH_SHSTK_SHSTK);
>  }
> +
> +/* We assume the prctl() feature bits line up with the arch_prctl() specific
> ones. */
> +static_assert(PR_SHADOW_STACK_ENABLE == ARCH_SHSTK_SHSTK);
> +static_assert(PR_SHADOW_STACK_WRITE  == ARCH_SHSTK_WRSS);

This is only needed for arch_get_shadow_stack_status()? So can we put it near by
and explain why?

> +
> +/* Handles the generic prctl interface for PR_SET_SHADOW_STACK_STATUS and its
> feature bits */
> +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
> +{
> +	int rc;

It needs to reject invalid options for status.

> +
> +	/* x86 arch_prctl is single bit at a time, so handle these one at
> time */
> +	if (!status & PR_SHADOW_STACK_ENABLE)
> +		return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_SHSTK);
> +
> +	rc = shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_SHSTK);
> +	if (rc)
> +		return rc;
> +
> +	if (status & PR_SHADOW_STACK_WRITE)
> +		return shstk_prctl(t, ARCH_SHSTK_ENABLE, ARCH_SHSTK_WRSS);
> +
> +	return shstk_prctl(t, ARCH_SHSTK_DISABLE, ARCH_SHSTK_WRSS);
> +}
> +
> +/* Handles the generic prctl interface for PR_LOCK_SHADOW_STACK_STATUS and
> its feature bits */
> +int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long
> status)
> +{
> +	return shstk_prctl(t, ARCH_SHSTK_LOCK, status);
> +}
> +
> +int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __user
> *status)
> +{
> +	return shstk_prctl(t, ARCH_SHSTK_STATUS, (unsigned long)status);
> +}