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

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Date:   Mon, 25 Jul 2022 10:57:36 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Dave Hansen <dave.hansen@...el.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 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.

Thanks,
Andrei