Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU_SPECTRE available for all architectures

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	"Colin Ian King" <colin.king@canonical.com>,
	"Frederic Weisbecker" <frederic@kernel.org>,
	"Mike Rapoport" <rppt@kernel.org>,
	"YiFei Zhu" <yifeifz2@illinois.edu>,
	"Steven Rostedt (VMware)" <rostedt@goodmis.org>,
	"Viresh Kumar" <viresh.kumar@linaro.org>,
	"Andrey Konovalov" <andreyknvl@gmail.com>,
	"Wang Kefeng" <wangkefeng.wang@huawei.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	bpf@vger.kernel.org
Subject: Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU_SPECTRE available for all architectures
Date: Fri, 29 Oct 2021 15:51:09 -0700	[thread overview]
Message-ID: <20211029225109.d3m2q4kuuzhzs2cv@gupta-dev2.localdomain> (raw)
In-Reply-To: <YXvIIwkfYcXEBf97@shell.armlinux.org.uk>

On 29.10.2021 11:08, Russell King (Oracle) wrote:
>On Thu, Oct 28, 2021 at 12:36:58PM -0700, Pawan Gupta wrote:
>> Isn't ARM already using CPU_SPECTRE for selecting things:
>>
>> 	config HARDEN_BRANCH_PREDICTOR
>> 	     bool "Harden the branch predictor against aliasing attacks" if EXPERT
>> 	     depends on CPU_SPECTRE
>>
>> This was the whole motivation for doing the same for x86.
>>
>> Adding a condition for all architectures is also okay, but its going to
>> a little messier:
>>
>> 	 config BPF_UNPRIV_DEFAULT_OFF
>> 	        default y if X86 || ARM || ...
>
>It doesn't have to be (but sadly we end up repeating "DEFAULT"):
>
>config BPF_UNPRIV_DEFAULT_OFF_DEFAULT
>	bool
>
>config BPF_UNPRIV_DEFAULT_OFF
>	bool "Disable unprivileged BPF by default"
>	default BPF_UNPRIV_DEFAULT_OFF_DEFAULT
>
>Then architectures can select BPF_UNPRIV_DEFAULT_OFF_DEFAULT if they
>wish this to be defaulted to "yes".

Looks like we are settling on unconditional 'default y' for now [1].
I have sent a v3 with 'default y' [2].

>However, please note that this has limited use given that the
>BPF_UNPRIV_DEFAULT_OFF option has been around for a while now. Any
>existing configuration that mentions this symbol will override any
>default specified in the Kconfig files if the option is user-visible.

Yes, existing configurations will have to toggle this manually. However,
many distros already have BPF_UNPRIV_DEFAULT_OFF=y in their
configuration.

>So, IMHO, defaults need to be set correctly from the point in time
>that the option is introduced.

Agree.

[1] https://lore.kernel.org/lkml/6130e55f-4d84-5ada-4e86-5b678e3eaf5e@iogearbox.net/
[2] https://lore.kernel.org/lkml/0ace9ce3f97656d5f62d11093ad7ee81190c3c25.1635535215.git.pawan.kumar.gupta@linux.intel.com/