linux-kernel - Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG

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Date:   Sun, 11 Sep 2022 01:22:52 +0200
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>
Subject: Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG_INVALID

On Wed, Sep 7, 2022 at 1:00 PM Vincenzo Frascino
<vincenzo.frascino@....com> wrote:
>
> When the kernel is entered on aarch64, the MTE allocation tags are in an
> UNKNOWN state.
>
> With MTE enabled, the tags are initialized:
>  - When a page is allocated and the user maps it with PROT_MTE.
>  - On allocation, with in-kernel MTE enabled (HW_TAGS KASAN).
>
> If the tag pool is zeroed by the hardware at reset, it makes it
> difficult to track potential places where the initialization of the
> tags was missed.
>
> This can be observed under QEMU for aarch64, which initializes the MTE
> allocation tags to zero.
>
> Initialize to tag storage to KASAN_TAG_INVALID to catch potential
> places where the initialization of the tags was missed.

Hi Vincenzo,

Cold you clarify what kind of places this refers to? Like the kernel
allocating memory and not setting the tags? Or is this related to
userspace applications? I'm not sure what's the user story for this
new flag is.

> This is done introducing a new kernel command line parameter
> "mte.tags_init" that enables the debug option.

Depending on the intended use, this can be extended to "mte.tags_init=<tag>".

> Note: The proposed solution should be considered a debug option because
> it might have performance impact on large machines at boot.

Thanks!