Re: [PATCH v6] ARM: Implement SLS mitigation

From: Will Deacon <will@kernel.org>
To: Jian Cai <jiancai@google.com>
Cc: ndesaulniers@google.com, manojgupta@google.com,
	llozano@google.com, clang-built-linux@googlegroups.com,
	"Nathan Chancellor" <nathan@kernel.org>,
	"David Laight" <David.Laight@aculab.com>,
	"Russell King" <rmk+kernel@armlinux.org.uk>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Russell King" <linux@armlinux.org.uk>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"James Morris" <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Andreas Färber" <afaerber@suse.de>,
	"Daniel Palmer" <daniel@0x0f.com>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Vladimir Murzin" <vladimir.murzin@arm.com>,
	"Marc Zyngier" <maz@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"David Brazdil" <dbrazdil@google.com>,
	"Joey Gouly" <joey.gouly@arm.com>,
	"James Morse" <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [PATCH v6] ARM: Implement SLS mitigation
Date: Fri, 5 Mar 2021 09:52:56 +0000	[thread overview]
Message-ID: <20210305095256.GA22536@willie-the-truck> (raw)
In-Reply-To: <20210305005327.405365-1-jiancai@google.com>

On Thu, Mar 04, 2021 at 04:53:18PM -0800, Jian Cai wrote:
> This patch adds CONFIG_HARDEN_SLS_ALL that can be used to turn on
> -mharden-sls=all, which mitigates the straight-line speculation
> vulnerability, speculative execution of the instruction following some
> unconditional jumps. Notice -mharden-sls= has other options as below,
> and this config turns on the strongest option.
> 
> all: enable all mitigations against Straight Line Speculation that are implemented.
> none: disable all mitigations against Straight Line Speculation.
> retbr: enable the mitigation against Straight Line Speculation for RET and BR instructions.
> blr: enable the mitigation against Straight Line Speculation for BLR instructions.
> 
> Links:
> https://reviews.llvm.org/D93221
> https://reviews.llvm.org/D81404
> https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/downloads/straight-line-speculation
> https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#SLS2
> 
> Suggested-by: Manoj Gupta <manojgupta@google.com>
> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: David Laight <David.Laight@aculab.com>
> Suggested-by: Will Deacon <will@kernel.org>

I'm still reasonably opposed to this patch, so please don't add my
"Suggested-by" here as, if I were to suggest anything, it would be not
to apply this patch :)

I still don't see why SLS is worth a compiler mitigation which will affect
all CPUs that run the kernel binary, but Spectre-v1 is not. In other words,
the big thing missing from this is a justification as to why SLS is a
problem worth working around for general C code.

Will