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

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Message-ID: <20200623153855.GM14101@suse.de>
Date:   Tue, 23 Jun 2020 17:38:55 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, 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>,
        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)

On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:
> > On Tue, Jun 23, 2020 at 04:53:44PM +0200, Peter Zijlstra wrote:
> > > +noinstr void idtentry_validate_ist(struct pt_regs *regs)
> > > +{
> > > +	if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
> > > +	    (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
> > > +		die("IST stack recursion", regs, 0);
> > > +}
> > 
> > Yes, this is a start, it doesn't cover the case where the NMI stack is
> > in-between, so I think you need to walk down regs->sp too.
> 
> That shouldn't be possible with the current code, I think.

Not with the current code, but possibly with SNP #VC exceptions:

      ->  First #VC
	  -> NMI before VC handler switched off its IST stack
	     (now on NMI IST stack)
	      -> Second SNP #VC exception before the NMI handler did the
		 #VC stack check (because HV messed around with some pages
		 touched there).

In the second #VC you use the same IST stack as in the first #VC, but
the the NMI-stack in-between.

> Reliability of that depends on the unwinder, I wouldn't want the guess
> uwinder to OOPS me by accident.

It doesn't use the full unwinder, it just assumes that there is a
pt_regs struct at the top of every kernel stack and walks through them
until SP points to a user-space stack.

As long as the assumption that there is a pt_regs struct on top of every
stack holds, this should be safe. The assumption might be wrong when an
exception happens during SYSCALL/SYSENTER entry, when the return frame
is not written by hardware.


	Joerg