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From:   Matthew Garrett <mjg59@google.com>
Date:   Mon, 2 Dec 2019 16:42:57 -0800
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Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot
To:     linux-efi <linux-efi@vger.kernel.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>, x86@kernel.org,
        linux-pci@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Dec 2, 2019 at 4:40 PM Matthew Garrett
<matthewgarrett@google.com> wrote:
>
> Add an option to disable the busmaster bit in the control register on
> all PCI bridges before calling ExitBootServices() and passing control to
> the runtime kernel. System firmware may configure the IOMMU to prevent
> malicious PCI devices from being able to attack the OS via DMA. However,
> since firmware can't guarantee that the OS is IOMMU-aware, it will tear
> down IOMMU configuration when ExitBootServices() is called. This leaves
> a window between where a hostile device could still cause damage before
> Linux configures the IOMMU again.

I don't know enough about ARM to know if this makes sense there as well. Anyone?