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Date:   Fri, 5 Mar 2021 09:52:56 +0000
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 =?iso-8859-1?Q?F=E4rber?= <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>,
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        <u.kleine-koenig@pengutronix.de>,
        Mark Rutland <mark.rutland@arm.com>,
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        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
Message-ID: <20210305095256.GA22536@willie-the-truck>
References: <20210223023542.2287529-1-jiancai@google.com>
 <20210305005327.405365-1-jiancai@google.com>
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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