kernel-hardening - Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code

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Date: Tue, 11 Feb 2020 09:54:02 +0000
From: Will Deacon <will@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: James Morse <james.morse@....com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Dave Martin <Dave.Martin@....com>,
	Kees Cook <keescook@...omium.org>,
	Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Jann Horn <jannh@...gle.com>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	clang-built-linux@...glegroups.com,
	kernel-hardening@...ts.openwall.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code

On Mon, Feb 10, 2020 at 06:24:32PM +0000, Mark Rutland wrote:
> On Mon, Feb 10, 2020 at 06:07:41PM +0000, Will Deacon wrote:
> > On Mon, Feb 10, 2020 at 06:03:28PM +0000, Mark Rutland wrote:
> > > On Mon, Feb 10, 2020 at 05:52:15PM +0000, Will Deacon wrote:
> > > > On Mon, Feb 10, 2020 at 05:18:58PM +0000, James Morse wrote:
> > > > > On 28/01/2020 18:49, Sami Tolvanen wrote:
> > > > > > Filter out CC_FLAGS_SCS and -ffixed-x18 for code that runs at a
> > > > > > different exception level.
> > > > > 
> > > > > Hmmm, there are two things being disabled here.
> > > > > 
> > > > > Stashing the lr in memory pointed to by VA won't work transparently at EL2 ... but
> > > > > shouldn't KVM's C code still treat x18 as a fixed register?
> > > > 
> > > > My review of v6 suggested dropping the -ffixed-x18 as well, since it's only
> > > > introduced by SCS (in patch 5) and so isn't required by anything else. Why
> > > > do you think it's needed?
> > > 
> > > When EL1 code calls up to hyp, it expects x18 to be preserved across the
> > > call, so hyp needs to either preserve it explicitly across a transitions
> > > from/to EL1 or always preserve it.
> > 
> > I thought we explicitly saved/restored it across the call after
> > af12376814a5 ("arm64: kvm: stop treating register x18 as caller save"). Is
> > that not sufficient?
> 
> That covers the hyp->guest->hyp round trip, but not the host->hyp->host
> portion surrounding that.

Thanks, I missed that. It's annoying that we'll end up needing /both/
-ffixed-x18 *and* the save/restore around guest transitions, but if we
actually want to use SCS for the VHE code then I see that it will be
required.

Sami -- can you restore -ffixed-x18 and then try the function attribute
as suggested by James, please?

Will

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