linux-kernel - Re: [RFC PATCH v4 19/27] x86/cet/shstk: Introduce WRUSS instruction

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Message-ID: <20181003041557.GA22724@asgard.redhat.com>
Date:   Wed, 3 Oct 2018 06:15:57 +0200
From:   Eugene Syromiatnikov <esyr@...hat.com>
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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        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>
Subject: Re: [RFC PATCH v4 19/27] x86/cet/shstk: Introduce WRUSS instruction

On Fri, Sep 21, 2018 at 08:03:43AM -0700, Yu-cheng Yu wrote:
> WRUSS is a new kernel-mode instruction but writes directly
> to user shadow stack memory.  This is used to construct
> a return address on the shadow stack for the signal
> handler.
> 
> This instruction can fault if the user shadow stack is
> invalid shadow stack memory.  In that case, the kernel does
> fixup.

"a fixup"

> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
>  arch/x86/mm/fault.c                  |  9 ++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 317fc59b512c..c04e68ef47da 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -237,6 +237,38 @@ 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 -1;

Should it...

> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	WARN_ONCE(1, "write_user_shstk_32 used but not supported.\n");

"is/was used"

> +	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 -1;

...and it be -EPERM, if -EFAULT was returned earlier for write_user_shstk_32?

> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 7c3877a982f4..4d4ac57a4ba2 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		error_code |= X86_PF_USER;
>  		flags |= FAULT_FLAG_USER;
>  	} else {
> +		/*
> +		 * WRUSS is a kernel instrcution and but writes

"WRUSS is a kernel instruction but writes"

> +		 * to user shadow stack.  When a fault occurs,
> +		 * both X86_PF_USER and X86_PF_SHSTK are set.
> +		 * Clear X86_PF_USER here.
> +		 */
> +		if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
> +		    (X86_PF_USER | X86_PF_SHSTK))
> +			error_code &= ~X86_PF_USER;
>  		if (regs->flags & X86_EFLAGS_IF)
>  			local_irq_enable();
>  	}
> -- 
> 2.17.1
>