linux-kernel - Re: [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction

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Date:   Thu, 30 Aug 2018 17:39:35 +0200
From:   Jann Horn <jannh@...gle.com>
To:     yu-cheng.yu@...el.com
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <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>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omiun.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>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction

On Thu, Aug 30, 2018 at 4:44 PM 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>
[...]
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +       int err = 0;
> +
> +       asm volatile("1: wrussq %1, (%0)\n"
> +                    "2:\n"
> +                    _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wruss)
> +                    :
> +                    : "r" (addr), "r" (val));
> +
> +       return err;
> +}

What's up with "err"? You set it to zero, and then you return it, but
nothing can ever set it to non-zero, right?

> +__visible bool ex_handler_wruss(const struct exception_table_entry *fixup,
> +                               struct pt_regs *regs, int trapnr)
> +{
> +       regs->ip = ex_fixup_addr(fixup);
> +       regs->ax = -1;
> +       return true;
> +}

And here you just write into regs->ax, but your "asm volatile" doesn't
reserve that register. This looks wrong to me.

I think you probably want to add something like an explicit
`"+&a"(err)` output to the asm statements.

> @@ -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

Nits: typo ("instrcution"), weird grammar ("and but writes")