linux-kernel - Re: [v2,03/11] arm64: Take into account ID_AA64PFR0

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Message-ID: <20180108182220.34233ebc@alans-desktop>
Date:   Mon, 8 Jan 2018 18:22:20 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Will Deacon <will.deacon@....com>
Cc:     Jayachandran C <jnair@...iumnetworks.com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, lorenzo.pieralisi@....com,
        ard.biesheuvel@...aro.org, catalin.marinas@....com,
        linux-kernel@...r.kernel.org, labbott@...hat.com,
        christoffer.dall@...aro.org
Subject: Re: [v2,03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3

> > On systems that are not vulnerable to variant 3, this is an unnecessary
> > overhead.  
> 
> KASLR can be bypassed on CPUs that are not vulnerable to variant 3 simply
> by timing how long accesses to kernel addresses from EL0 take -- please read
> the original KAISER paper for details about that attack on x86. kpti
> mitigates that. If you don't care about KASLR, don't enable it (arguably
> it's useless without kpti).

KASLR is primarily of value for remote protection.

Alan