linux-kernel - Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

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Date:   Tue, 23 Jun 2020 11:45:19 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <JGross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

Hi Andy,

On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
> 1. Use IST for #VC and deal with all the mess that entails.

With the removal of IST shifting I wonder what you would suggest on how
to best implement an NMI-safe IST handler with nesting support.

My current plan is to implement an IST handler which switches itself off
the IST stack as soon as possible, freeing it for re-use.

The flow would be roughly like this upon entering the handler;

	build_pt_regs();

	RSP = pt_regs->sp;

	if (RSP in VC_IST_stack)
		error("unallowed nesting")

	if (RSP in current_kernel_stack)
		RSP = round_down_to_8(RSP)
	else
		RSP = current_top_of_stack() // non-ist kernel stack

	copy_pt_regs(pt_regs, RSP);
	switch_stack_to(RSP);

To make this NMI safe, the NMI handler needs some logic too. Upon
entering NMI, it needs to check the return RSP, and if it is in the #VC
IST stack, it must do the above flow by itself and update the return RSP
and RIP. It needs to take into account the case when PT_REGS is not
fully populated on the return side.

Alternativly the NMI handler could safe/restore the contents of the #VC
IST stack or just switch to a special #VC-in-NMI IST stack.

All in all it could get complicated, and imho shift_ist would have been
simpler, but who am I anyway...

Or maybe you have a better idea how to implement this, so I'd like to
hear your opinion first before I spend too many days implementing
something.

Regards,

	Joerg