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

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Message-ID: <6f62b3c0-e796-e91c-f53b-23bd80fcb065@arm.com>
Date: Mon, 10 Feb 2020 17:18:58 +0000
From: James Morse <james.morse@....com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Will Deacon <will@...nel.org>, 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

Hi Sami,

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?

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.

Thanks,

James

> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> index ea710f674cb6..5843adef9ef6 100644
> --- a/arch/arm64/kvm/hyp/Makefile
> +++ b/arch/arm64/kvm/hyp/Makefile
> @@ -28,3 +28,6 @@ GCOV_PROFILE	:= n
>  KASAN_SANITIZE	:= n
>  UBSAN_SANITIZE	:= n
>  KCOV_INSTRUMENT	:= n
> +
> +# remove the SCS flags from all objects in this directory
> +KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))

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