linux-kernel - Re: [patch V4 09/65] x86/fpu: Sanitize xstateregs

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Message-ID: <291808ee-d5ab-a7b2-33e2-62a449e90cbf@intel.com>
Date:   Mon, 25 Jul 2022 14:26:03 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Chang Seok Bae <chang.seok.bae@...el.com>,
        Megha Dey <megha.dey@...ux.intel.com>,
        Oliver Sang <oliver.sang@...el.com>
Subject: Re: [patch V4 09/65] x86/fpu: Sanitize xstateregs_set()

On 7/25/22 10:57, Andrei Vagin wrote:
> On Mon, Jul 25, 2022 at 10:47 AM Dave Hansen <dave.hansen@...el.com> wrote:
>> On 7/13/22 21:04, Andrei Vagin wrote:
>>>> -       /*
>>>> -        * mxcsr reserved bits must be masked to zero for security reasons.
>>>> -        */
>>>> -       xsave->i387.mxcsr &= mxcsr_feature_mask;
>>>> -
>>>> -       /*
>>>> -        * In case of failure, mark all states as init:
>>>> -        */
>>>> -       if (ret)
>>>> -               fpstate_init(&fpu->state);
>>>> +       fpu__prepare_write(fpu);
>>>> +       ret = copy_kernel_to_xstate(&fpu->state.xsave, kbuf ?: tmpbuf);
>>> This change breaks gVisor. Now, when we set a new fpu state without
>>> fp,sse and ymm via ptrace, mxcsr isn't reset to the default value.
>>> The issue is reproduced only on hosts without xsaves. On hosts with
>>> xsaves, it works as expected.
>> Is gVisor some out-of-tree kernel code or is it just an proprietary KVM
>> user?  In other words, is this an issue with the upstream kernel itself
>> or does it require kernel modification?
> This is on the upstream kernel without any modifications. And this is the case
> when gVisor uses ptrace to trap syscalls. KVM isn't used in this case.

Do you happen to have a quick reproducer for this, or at least the
contents of the buffer that you are trying to restore?

I'm struggling to if we broke a meaningful part of the ABI, or if this
is fallout from the kernel being more hesitant to zap the whole buffer
when it has problems.