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

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Message-ID: <97273aa8-9aa4-78ee-21ca-d728835f44a7@intel.com>
Date:   Wed, 23 Sep 2020 15:53:46 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        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 9/23/20 3:47 PM, Andy Lutomirski wrote:
> 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.

Yeah, agreed.  I'd much rather make XRSTORS able to #GP gracefully than
teach the kernel exhaustively about every possible error condition it
can encounter.

We *might* want to do something like to preserve the warning if the task
hasn't been ptrace'd, or had the memory buffer written to directly or
tainted in another way.