linux-kernel - Re: [RFC PATCH v9 18/27] x86/cet/shstk: Introduce WRUSS instruction

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Date:   Tue, 25 Feb 2020 13:10:14 -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>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 18/27] x86/cet/shstk: Introduce WRUSS instruction

On Wed, Feb 05, 2020 at 10:19:26AM -0800, Yu-cheng Yu wrote:
> WRUSS is a new kernel-mode instruction but writes directly to user Shadow
> Stack (SHSTK) memory.  This is used to construct a return address on SHSTK
> for the signal handler.
> 
> This instruction can fault if the user SHSTK is not valid SHSTK memory.
> In that case, the kernel does a fixup.

Since these functions aren't used in this patch, should this get merged
with patch 19?

-Kees

> 
> v4:
> - Change to asm goto.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 6d37b8fcfc77..1b9b2e79c353 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -222,6 +222,38 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_INTEL_CET
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	asm_volatile_goto("1: wrussd %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	WARN_ONCE(1, "%s used but not supported.\n", __func__);
> +	return -EFAULT;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	asm_volatile_goto("1: wrussq %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  
> -- 
> 2.21.0
> 

-- 
Kees Cook