linux-kernel - Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction

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Message-ID: <20180711094448.GZ2476@hirez.programming.kicks-ass.net>
Date:   Wed, 11 Jul 2018 11:44:48 +0200
From:   Peter Zijlstra <peterz@...radead.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@...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@...omiun.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction

On Tue, Jul 10, 2018 at 03:26:30PM -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.
> 

> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	int err = 0;
> +
> +	asm volatile("1: wrussq %[val], (%[addr])\n"
> +		     "xor %[err], %[err]\n"

this XOR is superfluous, you already cleared @err above.

> +		     "2:\n"
> +		     ".section .fixup,\"ax\"\n"
> +		     "3: mov $-1, %[err]; jmp 2b\n"
> +		     ".previous\n"
> +		     _ASM_EXTABLE(1b, 3b)
> +		     : [err] "=a" (err)
> +		     : [val] "S" (val), [addr] "D" (addr));
> +
> +	return err;
> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")

What happened to:

  https://lkml.kernel.org/r/1528729376.4526.0.camel@2b52.sc.intel.com