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Message-ID: <1535646055.26689.10.camel@intel.com>
Subject: Re: [RFC PATCH v3 18/24] x86/cet/shstk: User-mode shadow stack
 support
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Jann Horn <jannh@google.com>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
Date:   Thu, 30 Aug 2018 09:20:55 -0700
In-Reply-To: <CAG48ez0d8+E_O-9u6ZHZ6dQO55Ui2ydV_kia-EEhyYeB_w4r2g@mail.gmail.com>
References: <20180830143904.3168-1-yu-cheng.yu@intel.com>
         <20180830143904.3168-19-yu-cheng.yu@intel.com>
         <CAG48ez0d8+E_O-9u6ZHZ6dQO55Ui2ydV_kia-EEhyYeB_w4r2g@mail.gmail.com>
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On Thu, 2018-08-30 at 18:10 +0200, Jann Horn wrote:
> On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@intel.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@intel.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)

The MSR is in the XSAVES buffer and switched by XSAVES/XRSTORS.

Yu-cheng