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

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Date:   Tue, 3 Dec 2019 07:30:06 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Matthew Garrett <matthewgarrett@...gle.com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        X86 ML <x86@...nel.org>, linux-pci@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot

On Mon, Dec 2, 2019 at 4:41 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.
>
> If CONFIG_EFI_NO_BUSMASTER is enabled or the "disable_busmaster=1"
> commandline argument is passed, the EFI stub will clear the busmaster
> bit on all PCI bridges before ExitBootServices() is called. This will
> prevent any malicious PCI devices from being able to perform DMA until
> the kernel reenables busmastering after configuring the IOMMU.

I hate to be an obnoxious bikeshedder, but I really dislike the
"disable_busmaster" name.  I read this and $SUBJECT as "for some
reason, the admin wants to operate the system with busmastering off".
What you really want is something more like "disable busmastering
before IOMMU initialization".  Maybe
"iommu.disable_busmaster_before_init"?

Similarly, EFI_NO_BUSMASTER sounds like a permanent state of affairs.

Would a similar patch apply to non-EFI boot?  That is, in a BIOS boot,
is busmastering on when the kernel is loaded?

--Andy