linux-kernel - Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

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Date:   Wed, 23 Sep 2020 15:47:12 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        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>
Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

On Wed, Sep 23, 2020 at 3:20 PM Yu, Yu-cheng <yu-cheng.yu@...el.com> wrote:
>
> On 9/23/2020 3:08 PM, Dave Hansen wrote:
> > On 9/23/20 3:06 PM, Yu, Yu-cheng wrote:
> >> I think I'll add a check here for (r + 8) >= TASK_SIZE_MAX.  It is
> >> better than getting a fault.
> >
> > There's also wrmsr_safe().
> >
> Yes, thanks.
>
> Since I am going to change this to:
>
> fpu__prepare_write(), then write to the XSAVES area.
>
> The kernel does not expect XRSTORS to fail ("Bad FPU state detected..."
> message).  So maybe still check the address first.

Surely there are plenty of ways to use ptrace() to poke garbage into
the FPU state.  We should be able to handle this type of failure
somewhat gracefully.