linux-kernel - Re: [PATCH 00/45] arm Spectre fix backport review for LTS 4.9

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Message-ID: <20180302103046.GC19323@arm.com>
Date:   Fri, 2 Mar 2018 10:30:46 +0000
From:   Will Deacon <will.deacon@....com>
To:     Alex Shi <alex.shi@...aro.org>
Cc:     Greg KH <greg@...ah.com>, Marc Zyngier <marc.zyngier@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        stable@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/45] arm Spectre fix backport review for LTS 4.9

On Fri, Mar 02, 2018 at 05:02:32PM +0800, Alex Shi wrote:
> As testing the spectre bug fix, that's a good question. I also asked
> this question to original patch authors, like Marc. They said they just
> figure out these patches could block spectre or meltdown issue. From my
> side, I just reproduced the process internal spectre. But all fix on arm
> can not resolve the user space internal spectre. It can block from user
> to kernel or kernel to user spectre according the code purose. So I
> believe these patch could do their job. And arm cpu would drop the
> spectre branches if it has 20+ 'nop' instructions...

Since this is archived on a public list and I don't want people to rely on
this, no, you cannot rely on "20+ 'nop' instructions" to work around
spectre on arm/arm64. It might prevent a particular PoC working on a
particular SoC, but it's fragile at best.

Will