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

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Date:   Mon, 10 Feb 2020 17:52:15 +0000
From:   Will Deacon <will@...nel.org>
To:     James Morse <james.morse@....com>
Cc:     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>,
        Mark Rutland <mark.rutland@....com>,
        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 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?

> As you have an __attribute__((no_sanitize("shadow-call-stack"))), could we add that to
> __hyp_text instead? (its a smaller hammer!) All of KVM's EL2 code is marked __hyp_text,
> but that isn't everything in these files. Doing it like this would leave KVM's VHE-only
> paths covered.
> 
> As an example, with VHE the kernel and KVM both run at EL2, and KVM behaves differently:
> kvm_vcpu_put_sysregs() in kvm/hyp/sysreg-sr.c is called from a preempt notifier as
> the EL2 registers are always accessible.

That's a good point, and I agree that it would be nice to have SCS covering
the VHE paths. If you do that as a function attribute (which feels pretty
fragile to me), then I guess we'll have to keep the -ffixed-x18 for the
non-VHE code after all because GCC at least doesn't like having the register
saving ABI specified on a per-function basis.

Will