linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: David Laight <David.Laight@ACULAB.COM>
To: 'Will Deacon' <will@kernel.org>, Jian Cai <jiancai@google.com>
Cc: "ndesaulniers@google.com" <ndesaulniers@google.com>,
	"manojgupta@google.com" <manojgupta@google.com>,
	"llozano@google.com" <llozano@google.com>,
	"clang-built-linux@googlegroups.com"
	<clang-built-linux@googlegroups.com>,
	"Nathan Chancellor" <nathan@kernel.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>,
	"Krzysztof Kozlowski" <krzk@kernel.org>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Andreas Färber" <afaerber@suse.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-security-module@vger.kernel.org"
	<linux-security-module@vger.kernel.org>
Subject: RE: [PATCH v2] ARM: Implement Clang's SLS mitigation
Date: Wed, 17 Feb 2021 11:05:38 +0000	[thread overview]
Message-ID: <a2b21c7326e94c63a64a2e29ae64ba3b@AcuMS.aculab.com> (raw)
In-Reply-To: <20210217094859.GA3706@willie-the-truck>

From: Will Deacon
> Sent: 17 February 2021 09:49
> 
> On Fri, Feb 12, 2021 at 11:52:53AM -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.
> 
> What exactly does this mitigation do? This should be documented somewhere,
> maybe in the Kconfig text?

I looked it up, it adds some fairly heavy serialising instructions
after the unconditional jump.
For BLR (call indirect) it has to use a BL (call) to an indirect jump.

I don't know if the execution of the serialising instructions
gets aborted.
If not you could end up with unexpected delays - like those on
some x86 cpu when they speculatively executed trig functions.

It all seems pretty broken though.
I'd expect the branch prediction unit to speculate at the jump
target for 'predicted taken' conditional jumps.
So you'd really expect unconditional jumps to behave the same way.
BLR ought to be using the branch target buffer (BTB).

(It isn't actually 100% clear that some processors don't use the BTB
for non-indirect jumps though....)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2021-02-17 11:10 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-12  5:14 [PATCH] " Jian Cai
2021-02-12  5:55 ` Nathan Chancellor
2021-02-12 10:41   ` David Laight
2021-02-12 19:52     ` [PATCH v2] " Jian Cai
2021-02-17  9:49       ` Will Deacon
2021-02-17 11:05         ` David Laight [this message]
2021-03-25 14:01         ` Linus Walleij
2021-02-17 18:20       ` Nick Desaulniers
2021-02-19 20:18       ` [PATCH v3] ARM: Implement " Jian Cai
2021-02-19 20:30         ` Nathan Chancellor
2021-02-19 23:08         ` [PATCH v4] " Jian Cai
2021-02-21 10:13           ` Russell King - ARM Linux admin
2021-02-22 11:58           ` Will Deacon
2021-02-22 21:50             ` Jian Cai
2021-02-23 10:04               ` Will Deacon
2021-03-03 15:18                 ` Linus Walleij
2021-03-03 15:29                   ` David Laight
2021-03-03 15:31                     ` Linus Walleij
2021-02-23  2:31           ` [PATCH v5] " Jian Cai
2021-02-23  2:35             ` Jian Cai
2021-03-03 15:04               ` Linus Walleij
2021-03-04 23:22                 ` Jian Cai
2021-03-06 12:25                   ` Linus Walleij
2021-03-10  4:43                     ` Jian Cai
2021-03-22 11:45                       ` Linus Walleij
2021-03-23 22:39                         ` Jian Cai
2021-03-05  0:53               ` [PATCH v6] " Jian Cai
2021-03-05  9:52                 ` Will Deacon
2021-03-06 12:27                   ` Linus Walleij

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=a2b21c7326e94c63a64a2e29ae64ba3b@AcuMS.aculab.com \
    --to=david.laight@aculab.com \
    --cc=afaerber@suse.de \
    --cc=ardb@kernel.org \
    --cc=arnd@arndb.de \
    --cc=catalin.marinas@arm.com \
    --cc=clang-built-linux@googlegroups.com \
    --cc=jiancai@google.com \
    --cc=jmorris@namei.org \
    --cc=keescook@chromium.org \
    --cc=krzk@kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-security-module@vger.kernel.org \
    --cc=linux@armlinux.org.uk \
    --cc=llozano@google.com \
    --cc=manojgupta@google.com \
    --cc=masahiroy@kernel.org \
    --cc=nathan@kernel.org \
    --cc=ndesaulniers@google.com \
    --cc=serge@hallyn.com \
    --cc=will@kernel.org \
    --subject='RE: [PATCH v2] ARM: Implement Clang'\''s SLS mitigation' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox