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From: Andy Lutomirski <luto@amacapital.net>
To: Matthew Garrett <matthewgarrett@google.com>
Cc: linux-efi <linux-efi@vger.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	X86 ML <x86@kernel.org>,
	linux-pci@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Matthew Garrett <mjg59@google.com>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges during boot
Date: Tue, 3 Dec 2019 07:30:06 -0800
Message-ID: <CALCETrWUYapn=vTbKnKFVQ3Y4vG0qHwux0ym_To2NWKPew+vrw@mail.gmail.com> (raw)
In-Reply-To: <20191203004043.174977-1-matthewgarrett@google.com>

On Mon, Dec 2, 2019 at 4:41 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.
>
> 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

  parent reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03  0:40 Matthew Garrett
2019-12-03  0:42 ` Matthew Garrett
2019-12-03 11:54   ` Ard Biesheuvel
2019-12-03 13:38     ` Laszlo Ersek
2019-12-03 19:36       ` Matthew Garrett
2019-12-03 19:40     ` Matthew Garrett
2019-12-04  7:11       ` Laszlo Ersek
2019-12-04 19:29         ` Matthew Garrett
2019-12-03 15:30 ` Andy Lutomirski [this message]
2019-12-03 16:33   ` Ard Biesheuvel
2019-12-03 19:41   ` Matthew Garrett
2019-12-04 19:50     ` Andy Lutomirski
2019-12-04 19:56       ` Matthew Garrett
2019-12-12 15:46         ` Ard Biesheuvel
2019-12-13 21:24           ` Matthew Garrett
2019-12-03 18:23 ` kbuild test robot
2019-12-05 13:04 ` kbuild test robot

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