linux-kernel - Re: [PATCH] KVM: arm64: Compute TPIDR

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Date:   Fri, 08 Jan 2021 16:51:11 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Steven Price <steven.price@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>
Subject: Re: [PATCH] KVM: arm64: Compute TPIDR_EL2 ignoring MTE tag

Hi Steven,

On 2021-01-08 16:12, Steven Price wrote:
> KASAN in HW_TAGS mode will store MTE tags in the top byte of the
> pointer. When computing the offset for TPIDR_EL2 we don't want anything
> in the top byte, so remove the tag to ensure the computation is correct
> no matter what the tag.
> 
> Fixes: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
> Signed-off-by: Steven Price <steven.price@....com>
> ---
> Without this fix I can't boot a config with KASAN_HW_TAGS and KVM on an
> MTE enabled host. I'm unsure if this should really be in
> this_cpu_ptr_nvhe_sym().

this_cpu_ptr_nvhe_sym() should return something that is valid for
the EL1 kernel, so I guess untagging in the helper may not be
that useful.

However, I'm more concerned by anything at requires us to follow
pointers set up by EL1 at EL2. It looks to me that the only reason
the whole thing works is because kern_hyp_va() *accidentally* drops
tags before applying the EL1/EL2 offset...

Or am I getting it wrong?

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...