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

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Date:   Mon, 25 Jul 2022 10:47:49 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrei Vagin <avagin@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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 7/13/22 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?