Re: [PATCH v4 39/39] x86/shstk: Add ARCH_SHSTK_STATUS

[Kees Cook]

On Fri, Dec 02, 2022 at 04:36:06PM -0800, Rick Edgecombe wrote:
> CRIU and GDB need to get the current shadow stack and WRSS enablement
> status. This information is already available via /proc/pid/status, but
> this is incovienent for CRIU because it involves parsing the text output
> in an area of the code where this is difficult. Provide a status
> arch_prctl(), ARCH_SHSTK_STATUS for retrieving the status. Have arg2 be a
> userspace address, and make the new arch_prctl simply copy the features
> out to userspace.
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Requested-by: Mike Rapoport <rppt@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> 
> v4:
>  - New patch
> 
>  Documentation/x86/shstk.rst       | 6 ++++++
>  arch/x86/include/asm/shstk.h      | 4 ++--
>  arch/x86/include/uapi/asm/prctl.h | 1 +
>  arch/x86/kernel/process_64.c      | 1 +
>  arch/x86/kernel/shstk.c           | 8 +++++++-
>  5 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/x86/shstk.rst b/Documentation/x86/shstk.rst
> index 0d7d1ccfff06..b3eb87046c27 100644
> --- a/Documentation/x86/shstk.rst
> +++ b/Documentation/x86/shstk.rst
> @@ -77,6 +77,11 @@ arch_prctl(ARCH_SHSTK_UNLOCK, unsigned long features)
>      Unlock features. 'features' is a mask of all features to unlock. All
>      bits set are processed, unset bits are ignored. Only works via ptrace.
>  
> +arch_prctl(ARCH_SHSTK_STATUS, unsigned long addr)
> +    Copy the currently enabled features to the address passed in addr. The
> +    features are described using the bits passed into the others in
> +    'features'.
> +
>  The return values are as following:
>      On success, return 0. On error, errno can be::
>  
> @@ -84,6 +89,7 @@ The return values are as following:
>          -EOPNOTSUPP if the feature is not supported by the hardware or
>           disabled by kernel parameter.
>          -EINVAL arguments (non existing feature, etc)
> +        -EFAULT if could not copy information back to userspace
>  
>  The feature's bits supported are::
>  
> diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
> index c82f22fd5e6d..23bfb63c597d 100644
> --- a/arch/x86/include/asm/shstk.h
> +++ b/arch/x86/include/asm/shstk.h
> @@ -15,7 +15,7 @@ struct thread_shstk {
>  	u64	size;
>  };
>  
> -long shstk_prctl(struct task_struct *task, int option, unsigned long features);
> +long shstk_prctl(struct task_struct *task, int option, unsigned long arg2);
>  void reset_thread_features(void);
>  int shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
>  			     unsigned long stack_size,
> @@ -31,7 +31,7 @@ static inline bool shstk_enabled(void)
>  }
>  #else
>  static inline long shstk_prctl(struct task_struct *task, int option,
> -			     unsigned long features) { return -EINVAL; }
> +			     unsigned long arg2) { return -EINVAL; }
>  static inline void reset_thread_features(void) {}
>  static inline int shstk_alloc_thread_stack(struct task_struct *p,
>  					   unsigned long clone_flags,
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 0c95688cf58e..abe3fe6db6d2 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -31,6 +31,7 @@
>  #define ARCH_SHSTK_DISABLE		0x5002
>  #define ARCH_SHSTK_LOCK			0x5003
>  #define ARCH_SHSTK_UNLOCK		0x5004
> +#define ARCH_SHSTK_STATUS		0x5005
>  
>  /* ARCH_SHSTK_ features bits */
>  #define ARCH_SHSTK_SHSTK		(1ULL <<  0)
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 2be6e01fb144..5dcf5426241b 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -922,6 +922,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>  	case ARCH_SHSTK_DISABLE:
>  	case ARCH_SHSTK_LOCK:
>  	case ARCH_SHSTK_UNLOCK:
> +	case ARCH_SHSTK_STATUS:
>  		return shstk_prctl(task, option, arg2);
>  	default:
>  		ret = -EINVAL;
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 95579f7bace3..05f8dcc19dbc 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -448,8 +448,14 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi
>  	return alloc_shstk(addr, aligned_size, size, set_tok);
>  }
>  
> -long shstk_prctl(struct task_struct *task, int option, unsigned long features)
> +long shstk_prctl(struct task_struct *task, int option, unsigned long arg2)
>  {
> +	unsigned long features = arg2;
> +
> +	if (option == ARCH_SHSTK_STATUS) {
> +		return put_user(task->thread.features, (unsigned long __user *)arg2);
> +	}

Doesn't need the braces, but that's fine by me.

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

> +
>  	if (option == ARCH_SHSTK_LOCK) {
>  		task->thread.features_locked |= features;
>  		return 0;
> -- 
> 2.17.1
> 

-- 
Kees Cook