linux-kernel - Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

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Message-Id: <99B32E59-CFF2-4756-89BD-AEA0021F355F@amacapital.net>
Date:   Fri, 25 Sep 2020 09:51:14 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <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>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.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@...omium.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation



> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@...el.com> wrote:
> 
> On 9/25/2020 9:31 AM, Andy Lutomirski wrote:
>>> On Fri, Sep 25, 2020 at 7:58 AM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>>> 
> 
> [...]
> 
>>> @@ -286,6 +289,37 @@ bool emulate_vsyscall(unsigned long error_code,
>>>         /* Emulate a ret instruction. */
>>>         regs->ip = caller;
>>>         regs->sp += 8;
>>> +
>>> +#ifdef CONFIG_X86_CET
>>> +       if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) {
>>> +               struct cet_user_state *cet;
>>> +               struct fpu *fpu;
>>> +
>>> +               fpu = &tsk->thread.fpu;
>>> +               fpregs_lock();
>>> +
>>> +               if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
>>> +                       copy_fpregs_to_fpstate(fpu);
>>> +                       set_thread_flag(TIF_NEED_FPU_LOAD);
>>> +               }
>>> +
>>> +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>>> +               if (!cet) {
>>> +                       fpregs_unlock();
>>> +                       goto sigsegv;
>> I *think* your patchset tries to keep cet.shstk_size and
>> cet.ibt_enabled in sync with the MSR, in which case it should be
>> impossible to get here, but a comment and a warning would be much
>> better than a random sigsegv.
> 
> Yes, it should be impossible to get here.  I will add a comment and a warning, but still do sigsegv.  Should this happen, and the function return, the app gets a control-protection fault.  Why not let it fail early?

I’m okay with either approach as long as we get a comment and warning.

> 
>> 
>> Shouldn't we have a get_xsave_addr_or_allocate() that will never
>> return NULL but instead will mark the state as in use and set up the
>> init state if the feature was previously not in use?
> 
> We already have a static __raw_xsave_addr(), which returns a pointer to the requested xstate.  Maybe we can export __raw_xsave_addr(), if that is needed.

I don’t think that’s what we want in general — we want the whole construct of initializing the state if needed.