linux-kernel - Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

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Date:   Thu, 7 Jun 2018 09:40:02 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@...el.com> 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.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/include/asm/special_insns.h          | 44 +++++++++++++++++++++++++++
>  arch/x86/lib/x86-opcode-map.txt               |  2 +-
>  arch/x86/mm/fault.c                           | 13 +++++++-
>  tools/objtool/arch/x86/lib/x86-opcode-map.txt |  2 +-
>  4 files changed, 58 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 317fc59b512c..8ce532fcc171 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -237,6 +237,50 @@ 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)
> +{
> +       int err;
> +

Please add a comment indicating what exact opcode this is.

Peterz, isn't there some fancy better way we're supposed to handle the
error return these days?

> +       asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"
> +                    "xor %[err],%[err]\n"
> +                    "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)
> +               : "memory");
> +       return err;
> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +       return 0;

BUG()?  Or just omit the ifdef?  It seems unhelpful to have a stub
function that does nothing.

> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +       int err;
> +

Comment here too, please.

> +       asm volatile("1:.byte 0x66, 0x48, 0x0f, 0x38, 0xf5, 0x37\n"
> +                    "xor %[err],%[err]\n"
> +                    "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)
> +               : "memory");
> +       return err;
> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>
>