linux-kernel - Re: [PATCH v4] arm64: mte: move register initialization to C

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Date:   Fri, 9 Sep 2022 17:54:13 -0700
From:   Peter Collingbourne <pcc@...gle.com>
To:     Evgenii Stepanov <eugenis@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Kenny Root <kroot@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Will Deacon <will@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v4] arm64: mte: move register initialization to C

On Tue, Sep 6, 2022 at 5:36 PM Evgenii Stepanov <eugenis@...gle.com> wrote:
>
> If FEAT_MTE2 is disabled via the arm64.nomte command line argument on a
> CPU that claims to support FEAT_MTE2, the kernel will use Tagged Normal
> in the MAIR. If we interpret arm64.nomte to mean that the CPU does not
> in fact implement FEAT_MTE2, setting the system register like this may
> lead to UNSPECIFIED behavior. Fix it by arranging for MAIR to be set
> in the C function cpu_enable_mte which is called based on the sanitized
> version of the system register.
>
> There is no need for the rest of the MTE-related system register
> initialization to happen from assembly, with the exception of TCR_EL1,
> which must be set to include at least TBI1 because the secondary CPUs
> access KASan-allocated data structures early. Therefore, make the TCR_EL1
> initialization unconditional and move the rest of the initialization to
> cpu_enable_mte so that we no longer have a dependency on the unsanitized
> ID register value.

Moving the register initialization to C also fixes a bug where the
kernel's zeroing of TFSR_EL1 has no practical effect when the kernel
is started in VHE mode because the register is currently being zeroed
prior to the kernel enabling the redirect of TFSR_EL2 to TFSR_EL1 when
it enables VHE. As a result, without this patch it is possible to get
a spurious KASAN error report if TFSR_EL2 is non-zero out of reset.

Peter