linux-kernel - Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges during boot

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Message-ID: <CACdnJus7nHdr4p4H1j5as9eB=FG-uX+wy_tjvTQ5ObErDJHdow@mail.gmail.com>
Date:   Mon, 2 Dec 2019 16:42:57 -0800
From:   Matthew Garrett <mjg59@...gle.com>
To:     linux-efi <linux-efi@...r.kernel.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>, x86@...nel.org,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot

On Mon, Dec 2, 2019 at 4:40 PM Matthew Garrett
<matthewgarrett@...gle.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?