linux-kernel - Re: [PATCH v16 1/7] arm64: mte: Handle race when synchronising tags

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Message-ID: <20210618144013.GE16116@arm.com>
Date:   Fri, 18 Jun 2021 15:40:13 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Steven Price <steven.price@....com>
Cc:     Marc Zyngier <maz@...nel.org>, 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, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v16 1/7] arm64: mte: Handle race when synchronising tags

On Fri, Jun 18, 2021 at 02:28:20PM +0100, Steven Price wrote:
> mte_sync_tags() used test_and_set_bit() to set the PG_mte_tagged flag
> before restoring/zeroing the MTE tags. However if another thread were to
> race and attempt to sync the tags on the same page before the first
> thread had completed restoring/zeroing then it would see the flag is
> already set and continue without waiting. This would potentially expose
> the previous contents of the tags to user space, and cause any updates
> that user space makes before the restoring/zeroing has completed to
> potentially be lost.
> 
> Since this code is run from atomic contexts we can't just lock the page
> during the process. Instead implement a new (global) spinlock to protect
> the mte_sync_page_tags() function.
> 
> Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Steven Price <steven.price@....com>

Although I reviewed this patch, I think we should drop it from this
series and restart the discussion with the Chromium guys on what/if they
need PROT_MTE with MAP_SHARED. It currently breaks if you have two
PROT_MTE mappings but if they are ok with only one of the mappings being
PROT_MTE, I'm happy to just document it.

Not sure whether subsequent patches depend on it though.

-- 
Catalin