linux-kernel - Re: [PATCH 2/2] arm64: errata: Add Cortex-A520 speculative unprivileged load workaround

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Message-ID: <dcb12b18edc02634be3ac9909fa70602@misterjones.org>
Date:   Mon, 18 Sep 2023 11:17:46 +0100
From:   Marc Zyngier <maz@...terjones.org>
To:     Rob Herring <robh@...nel.org>, Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Jonathan Corbet <corbet@....net>,
        James Morse <james.morse@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64: errata: Add Cortex-A520 speculative
 unprivileged load workaround

On 2023-09-18 11:01, Will Deacon wrote:
> On Tue, Sep 12, 2023 at 07:11:15AM -0500, Rob Herring wrote:
>> Implement the workaround for ARM Cortex-A520 erratum 2966298. On an
>> affected Cortex-A520 core, a speculatively executed unprivileged load
>> might leak data from a privileged level via a cache side channel.
>> 
>> The workaround is to execute a TLBI before returning to EL0. A
>> non-shareable TLBI to any address is sufficient.
> 
> Can you elaborate at all on how this works, please? A TLBI addressing a
> cache side channel feels weird (or is "cache" referring to some TLB
> structures rather than e.g. the data cache here?).
> 
> Assuming there's some vulnerable window between the speculative
> unprivileged load and the completion of the TLBI, what prevents another
> CPU from observing the side-channel during that time? Also, does the
> TLBI need to be using the same ASID as the unprivileged load? If so, 
> then
> a context-switch could widen the vulnerable window quite significantly.

Another 'interesting' case is the KVM world switch. If EL0 is
affected, what about EL1? Can such a data leak exist cross-EL1,
or from EL2 to El1? Asking for a friend...

         M.
-- 
Who you jivin' with that Cosmik Debris?