Re: [PATCH 05/12] PCI: Require CAP_COMPROMISE_KERNEL for PCI BAR access

[Josh Boyer <jwboyer@gmail.com>]

On Mon, Mar 18, 2013 at 5:32 PM, Matthew Garrett
<matthew.garrett@nebula.com> wrote:
> Any hardware that can potentially generate DMA has to be locked down from
> userspace in order to avoid it being possible for an attacker to cause
> arbitrary kernel behaviour. Default to paranoid - in future we can
> potentially relax this for sufficiently IOMMU-isolated devices.
>
> Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>

As noted here:

https://bugzilla.redhat.com/show_bug.cgi?id=908888

this breaks pci passthru with QEMU.  The suggestion in the bug is to move
the check from read/write to open, but sysfs makes that somewhat
difficult.  The open code is part of the core sysfs functionality shared
with the majority of sysfs files, so adding a check there would restrict
things that clearly don't need to be restricted.

Kyle had the idea to add a cap field to the attribute structure, and do
a capable check if that is set.  That would allow for a more generic
usage of capabilities in sysfs code, at the cost of slightly increasing
the structure size and open path.  That seems somewhat promising if we
stick with capabilities.

I would love to just squarely blame capabilities for causing this, but we
can't just replace it with an efi_enabled(EFI_SECURE_BOOT) check because
of the sysfs open case.  I'm not sure there are great answers here.

josh