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

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Date:   Mon, 7 Nov 2022 15:19:30 +0000
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG_INVALID

On Wed, Sep 07, 2022 at 12:00:15PM +0100, Vincenzo Frascino 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.
> 
> This is done introducing a new kernel command line parameter
> "mte.tags_init" that enables the debug option.
> 
> Note: The proposed solution should be considered a debug option because
> it might have performance impact on large machines at boot.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
>  arch/arm64/kernel/mte.c | 47 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)

I don't really see the point in this change -- who is going to use this
option?

Will