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

Kees Cook
2022-12-03T02:57:30Z

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@intel.com>
> Requested-by: Mike Rapoport <rppt@kernel.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
>
> 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@chromium.org>

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

--
Kees Cook