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

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Message-ID: <20201102183938.GA10749@zn.tnic>
Date:   Mon, 2 Nov 2020 19:39:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        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 01, 2020 at 03:04:48PM -0700, Andy Lutomirski wrote:
> On Thu, Oct 1, 2020 at 2:50 PM Dave Hansen <dave.hansen@...el.com> wrote:
> > 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.

Just for the record, I like this: safe and simple. We can always do
smarter shenanigans later, if at all needed, that is.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette