linux-kernel - Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

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Date:   Wed, 24 Mar 2021 14:30:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>, Len Brown <lenb@...nel.org>
Cc:     "Liu, Jing2" <jing2.liu@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        X86 ML <x86@...nel.org>, Len Brown <len.brown@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the
 first use of dynamic user state

On 3/24/21 2:26 PM, Andy Lutomirski wrote:
>> 3. user space always uses fully uncompacted XSAVE buffers.
>> 
> There is no reason we have to do this for new states. Arguably we
> shouldn’t for AMX to avoid yet another altstack explosion.

The thing that's worried me is that the list of OS-enabled states is
visible to apps via XGETBV.  It doesn't seem too much of a stretch to
think that apps will see AMX enabled with XGETBV and them assume that
it's on the signal stack.

Please tell me I'm being too paranoid.  If we can break this assumption,
it would get rid of a lot of future pain.