linux-kernel - Re: [PATCH] kvm: Ensure writes to the coalesced MMIO ring are within bounds

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Message-ID: <281758dd-e400-f99c-b2e2-d32bca9f371e@redhat.com>
Date:   Wed, 18 Sep 2019 16:11:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     kvm@...r.kernel.org, kernellwp@...il.com,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "# 5 . 2 . y" <stable@...nel.org>
Subject: Re: [PATCH] kvm: Ensure writes to the coalesced MMIO ring are within
 bounds

On 18/09/19 15:59, Will Deacon wrote:
> Okey doke, as long as it gets fixed! My minor concerns with the error-checking
> variant are:
> 
>   * Whether or not you need a READ_ONCE to prevent the compiler potentially
>     reloading 'ring->last' after validation

Yes, it certainly needs READ_ONCE.  I had already added it locally, indeed.

>   * Whether or not this could be part of a spectre-v1 gadget

I think not, the spectrev1 gadget require a load that is indexed from
the contents of ring->coalesced_mmio[insert], but there's none.

Paolo

> so, given that I don't think the malicious host deserves an error code if it
> starts writing the 'last' index, I went with the "obviously safe" version.
> But up to you.
> 
> Will
>