linux-kernel - Re: How should we handle illegal task FPU state?

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Date:   Thu, 1 Oct 2020 15:04:48 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: How should we handle illegal task FPU state?

On Thu, Oct 1, 2020 at 2:50 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 10/1/20 1:58 PM, Sean Christopherson wrote:
> > One thought for a lowish effort approach to pave the way for CET would be to
> > try XRSTORS multiple times in switch_fpu_return().  If the first try fails,
> > then WARN, init non-supervisor state and try a second time, and if _that_ fails
> > then kill the task.  I.e. do the minimum effort to play nice with bad FPU
> > state, but don't let anything "accidentally" turn off CET.
>
> I'm not sure we should ever keep running userspace after an XRSTOR*
> failure.  For MPX, this might have provided a nice, additional vector
> for an attacker to turn off MPX.  Same for pkeys if we didn't correctly
> differentiate between the hardware init state versus the "software init"
> state that we keep in init_task.
>
> What's the advantage of letting userspace keep running after we init its
> state?  That it _might_ be able to recover?

I suppose we can kill userspace and change that behavior only if
someone complains.  I still think it would be polite to try to dump
core, but that could be tricky with the current code structure.  I'll
try to whip up a patch.  Maybe I'll add a debugfs file to trash MXCSR
for testing.