linux-kernel - Re: [PATCH v19 24/25] x86/cet/shstk: Add arch

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Date:   Thu, 4 Feb 2021 12:35:39 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v19 24/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

On Wed, Feb 03, 2021 at 02:55:46PM -0800, Yu-cheng Yu wrote:
> arch_prctl(ARCH_X86_CET_STATUS, u64 *args)
>     Get CET feature status.
> 
>     The parameter 'args' is a pointer to a user buffer.  The kernel returns
>     the following information:
> 
>     *args = shadow stack/IBT status
>     *(args + 1) = shadow stack base address
>     *(args + 2) = shadow stack size

What happens if this needs to grow in the future? Should the first u64
contain the array size?

Otherwise, looks sensible.

-Kees

> 
>     32-bit binaries use the same interface, but only lower 32-bits of each
>     item.
> 
> arch_prctl(ARCH_X86_CET_DISABLE, unsigned int features)
>     Disable CET features specified in 'features'.  Return -EPERM if CET is
>     locked.
> 
> arch_prctl(ARCH_X86_CET_LOCK)
>     Lock in CET features.
> 
> Also change do_arch_prctl_common()'s parameter 'cpuid_enabled' to
> 'arg2', as it is now also passed to prctl_cet().
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/include/asm/cet.h        |  3 ++
>  arch/x86/include/uapi/asm/prctl.h |  4 +++
>  arch/x86/kernel/Makefile          |  2 +-
>  arch/x86/kernel/cet_prctl.c       | 60 +++++++++++++++++++++++++++++++
>  arch/x86/kernel/process.c         |  6 ++--
>  5 files changed, 71 insertions(+), 4 deletions(-)
>  create mode 100644 arch/x86/kernel/cet_prctl.c
> 
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> index ec4b5e62d0ce..16870e5bc8eb 100644
> --- a/arch/x86/include/asm/cet.h
> +++ b/arch/x86/include/asm/cet.h
> @@ -14,9 +14,11 @@ struct sc_ext;
>  struct cet_status {
>  	unsigned long	shstk_base;
>  	unsigned long	shstk_size;
> +	unsigned int	locked:1;
>  };
>  
>  #ifdef CONFIG_X86_CET
> +int prctl_cet(int option, u64 arg2);
>  int cet_setup_shstk(void);
>  int cet_setup_thread_shstk(struct task_struct *p, unsigned long clone_flags);
>  void cet_disable_shstk(void);
> @@ -25,6 +27,7 @@ int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp)
>  void cet_restore_signal(struct sc_ext *sc);
>  int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
>  #else
> +static inline int prctl_cet(int option, u64 arg2) { return -EINVAL; }
>  static inline int cet_setup_thread_shstk(struct task_struct *p,
>  					 unsigned long clone_flags) { return 0; }
>  static inline void cet_disable_shstk(void) {}
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 5a6aac9fa41f..9245bf629120 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -14,4 +14,8 @@
>  #define ARCH_MAP_VDSO_32	0x2002
>  #define ARCH_MAP_VDSO_64	0x2003
>  
> +#define ARCH_X86_CET_STATUS		0x3001
> +#define ARCH_X86_CET_DISABLE		0x3002
> +#define ARCH_X86_CET_LOCK		0x3003
> +
>  #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 4a9a7e7d00dc..2f60a28769f9 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -151,7 +151,7 @@ obj-$(CONFIG_UNWINDER_FRAME_POINTER)	+= unwind_frame.o
>  obj-$(CONFIG_UNWINDER_GUESS)		+= unwind_guess.o
>  
>  obj-$(CONFIG_AMD_MEM_ENCRYPT)		+= sev-es.o
> -obj-$(CONFIG_X86_CET)			+= cet.o
> +obj-$(CONFIG_X86_CET)			+= cet.o cet_prctl.o
>  
>  ###
>  # 64 bit specific files
> diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
> new file mode 100644
> index 000000000000..0030c63a08c0
> --- /dev/null
> +++ b/arch/x86/kernel/cet_prctl.c
> @@ -0,0 +1,60 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/errno.h>
> +#include <linux/uaccess.h>
> +#include <linux/prctl.h>
> +#include <linux/compat.h>
> +#include <linux/mman.h>
> +#include <linux/elfcore.h>
> +#include <linux/processor.h>
> +#include <asm/prctl.h>
> +#include <asm/cet.h>
> +
> +/* See Documentation/x86/intel_cet.rst. */
> +
> +static int cet_copy_status_to_user(struct cet_status *cet, u64 __user *ubuf)
> +{
> +	u64 buf[3] = {};
> +
> +	if (cet->shstk_size) {
> +		buf[0] |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
> +		buf[1] = cet->shstk_base;
> +		buf[2] = cet->shstk_size;
> +	}
> +
> +	return copy_to_user(ubuf, buf, sizeof(buf));
> +}
> +
> +int prctl_cet(int option, u64 arg2)
> +{
> +	struct cet_status *cet;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_CET))
> +		return -ENOTSUPP;
> +
> +	cet = &current->thread.cet;
> +
> +	if (option == ARCH_X86_CET_STATUS)
> +		return cet_copy_status_to_user(cet, (u64 __user *)arg2);
> +
> +	switch (option) {
> +	case ARCH_X86_CET_DISABLE:
> +		if (cet->locked)
> +			return -EPERM;
> +
> +		if (arg2 & ~GNU_PROPERTY_X86_FEATURE_1_VALID)
> +			return -EINVAL;
> +		if (arg2 & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
> +			cet_disable_shstk();
> +		return 0;
> +
> +	case ARCH_X86_CET_LOCK:
> +		if (arg2)
> +			return -EINVAL;
> +		cet->locked = 1;
> +		return 0;
> +
> +	default:
> +		return -ENOSYS;
> +	}
> +}
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 3af6b36e1a5c..9e11e5f589f3 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -979,14 +979,14 @@ unsigned long get_wchan(struct task_struct *p)
>  }
>  
>  long do_arch_prctl_common(struct task_struct *task, int option,
> -			  unsigned long cpuid_enabled)
> +			  unsigned long arg2)
>  {
>  	switch (option) {
>  	case ARCH_GET_CPUID:
>  		return get_cpuid_mode();
>  	case ARCH_SET_CPUID:
> -		return set_cpuid_mode(task, cpuid_enabled);
> +		return set_cpuid_mode(task, arg2);
>  	}
>  
> -	return -EINVAL;
> +	return prctl_cet(option, arg2);
>  }
> -- 
> 2.21.0
> 
> 

-- 
Kees Cook