linux-kernel - Re: [RFC PATCH v3 18/24] x86/cet/shstk: User-mode shadow stack support

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Message-ID: <CAG48ez0d8+E_O-9u6ZHZ6dQO55Ui2ydV_kia-EEhyYeB_w4r2g@mail.gmail.com>
Date:   Thu, 30 Aug 2018 18:10:17 +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 18/24] x86/cet/shstk: User-mode shadow stack support

On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>
> This patch adds basic shadow stack enabling/disabling routines.
> A task's shadow stack is allocated from memory with VM_SHSTK
> flag set and read-only protection.  The shadow stack is
> allocated to a fixed size of RLIMIT_STACK.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
[...]
> +static int set_shstk_ptr(unsigned long addr)
> +{
> +       u64 r;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +               return -1;
> +
> +       if ((addr >= TASK_SIZE_MAX) || (!IS_ALIGNED(addr, 4)))
> +               return -1;
> +
> +       rdmsrl(MSR_IA32_U_CET, r);
> +       wrmsrl(MSR_IA32_PL3_SSP, addr);
> +       wrmsrl(MSR_IA32_U_CET, r | MSR_IA32_CET_SHSTK_EN);
> +       return 0;
> +}

Here's a really stupid question: Where is the logic for switching
those MSRs on task switch? MSR_IA32_PL3_SSP contains a userspace
pointer, so it has to be switched on task switch, right? I'm sure I'm
missing something obvious, but grepping for places that set
MSR_IA32_PL3_SSP to nonzero values through the entire patchset, I only
see set_shstk_ptr(), which is called from:

 - cet_setup_shstk() (called from arch_setup_features(), which is
called from load_elf_binary())
 - cet_restore_signal() (called on signal handler return)
 - cet_setup_signal() (called from signal handling code)