Re: [PATCH v7 28/41] x86: Introduce userspace API for shadow stack

[Borislav Petkov]

On Mon, Feb 27, 2023 at 02:29:44PM -0800, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> Add three new arch_prctl() handles:
> 
>  - ARCH_SHSTK_ENABLE/DISABLE enables or disables the specified
>    feature. Returns 0 on success or an error.

"... or a negative value on error."

>  - ARCH_SHSTK_LOCK prevents future disabling or enabling of the
>    specified feature. Returns 0 on success or an error

ditto.

What is the use case of the feature locking?

I'm under the simple assumption that once shstk is enabled for an app,
it remains so. I guess my question is rather, what's the use case for
enabling shadow stack and then disabling it later for an app...?

> The features are handled per-thread and inherited over fork(2)/clone(2),
> but reset on exec().
> 
> This is preparation patch. It does not implement any features.

That belongs under the "---" line I guess.

> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> [tweaked with feedback from tglx]
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> 
> ---
> v4:
>  - Remove references to CET and replace with shadow stack (Peterz)
> 
> v3:
>  - Move shstk.c Makefile changes earlier (Kees)
>  - Add #ifdef around features_locked and features (Kees)
>  - Encapsulate features reset earlier in reset_thread_features() so
>    features and features_locked are not referenced in code that would be
>    compiled !CONFIG_X86_USER_SHADOW_STACK. (Kees)
>  - Fix typo in commit log (Kees)
>  - Switch arch_prctl() numbers to avoid conflict with LAM
> 
> v2:
>  - Only allow one enable/disable per call (tglx)
>  - Return error code like a normal arch_prctl() (Alexander Potapenko)
>  - Make CET only (tglx)
> ---
>  arch/x86/include/asm/processor.h  |  6 +++++
>  arch/x86/include/asm/shstk.h      | 21 +++++++++++++++
>  arch/x86/include/uapi/asm/prctl.h |  6 +++++
>  arch/x86/kernel/Makefile          |  2 ++
>  arch/x86/kernel/process_64.c      |  7 ++++-
>  arch/x86/kernel/shstk.c           | 44 +++++++++++++++++++++++++++++++
>  6 files changed, 85 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/include/asm/shstk.h
>  create mode 100644 arch/x86/kernel/shstk.c

...

> +long shstk_prctl(struct task_struct *task, int option, unsigned long features)
> +{
> +	if (option == ARCH_SHSTK_LOCK) {
> +		task->thread.features_locked |= features;
> +		return 0;
> +	}
> +
> +	/* Don't allow via ptrace */
> +	if (task != current)
> +		return -EINVAL;
> +
> +	/* Do not allow to change locked features */
> +	if (features & task->thread.features_locked)
> +		return -EPERM;
> +
> +	/* Only support enabling/disabling one feature at a time. */
> +	if (hweight_long(features) > 1)
> +		return -EINVAL;
> +
> +	if (option == ARCH_SHSTK_DISABLE) {
> +		return -EINVAL;
> +	}

{} braces left over from some previous version. Can go now.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette