linux-kernel - Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci

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Date:	Tue, 6 Oct 2015 19:09:11 +0300
From:	Avi Kivity <avi@...lladb.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Avi Kivity <avi@...udius-systems.com>
Cc:	Vlad Zolotarov <vladz@...udius-systems.com>,
	linux-kernel@...r.kernel.org, hjk@...sjkoch.de, corbet@....net,
	gregkh@...uxfoundation.org, bruce.richardson@...el.com,
	gleb@...udius-systems.com, stephen@...workplumber.org,
	alexander.duyck@...il.com
Subject: Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci_generic
 driver



On 10/06/2015 06:15 PM, Michael S. Tsirkin wrote:
>> While it is possible that userspace malfunctions and accidentally programs
>> MSI incorrectly, the risk is dwarfed by the ability of userspace to program
>> DMA incorrectly.
> That seems to imply that for the upstream kernel this is not a valid usecase at all.
>

That is trivially incorrect, upstream pci_uio_generic is used with dpdk 
for years.  Are dpdk applications an invalid use case?

Again:

- security is not compromised.  you need to be root to (ab)use this.
- all of the potentially compromising functionality has been there from 
day 1
- uio_pci_generic is the only way to provide the required performance on 
some configurations (where kernel drivers, or userspace drivers + iommu 
are too slow)
- uio_pci_generic + msix is the only way to enable userspace drivers on 
some configurations (SRIOV)

The proposed functionality does not increase the attack surface.
The proposed functionality marginally increases the bug surface.
The proposed functionality is a natural evolution of uio_pci_generic.

There is a new class of applications (network function virtualization) 
which require this.  They can't use the kernel drivers because they are 
too slow.  They can't use the iommu because it is either too slow, or 
taken over by the hypervisor.  They are willing to live with less kernel 
protection, because they are a single user application anyway (and since 
they use a kernel bypass, they don't really care that much about the 
kernel).

The kernel serves more use-cases than a desktop or a multi-user 
servers.  Some of these users are willing to trade off protection for 
performance or functionality (an extreme, yet similar, example is 
linux-nommu, which allows any application to access any bit of memory, 
due to the lack of protection hardware).
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