linux-kernel - Re: [PATCH mm v10 26/42] arm64: mte: Reset the page tag in page->flags

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Message-ID: <20201114124346.GC2837@gaia>
Date:   Sat, 14 Nov 2020 12:43:46 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        kasan-dev@...glegroups.com, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm v10 26/42] arm64: mte: Reset the page tag in
 page->flags

On Fri, Nov 13, 2020 at 11:15:54PM +0100, Andrey Konovalov wrote:
> From: Vincenzo Frascino <vincenzo.frascino@....com>
> 
> The hardware tag-based KASAN for compatibility with the other modes
> stores the tag associated to a page in page->flags.
> Due to this the kernel faults on access when it allocates a page with an
> initial tag and the user changes the tags.
> 
> Reset the tag associated by the kernel to a page in all the meaningful
> places to prevent kernel faults on access.
> 
> Note: An alternative to this approach could be to modify page_to_virt().
> This though could end up being racy, in fact if a CPU checks the
> PG_mte_tagged bit and decides that the page is not tagged but another
> CPU maps the same with PROT_MTE and becomes tagged the subsequent kernel
> access would fail.
> 
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>