Re: [PATCH 2/3] PCI: Handle device quirks when accessing sysfs resource<N>entries

From: Robert Hancock
Date: Thu Mar 21 2013 - 20:51:52 EST

On 03/20/2013 10:35 PM, Myron Stowe wrote:
Sysfs includes entries to memory regions that back a PCI device's BARs.
The pci-sysfs entries backing I/O Port BARs can be accessed by userspace,
providing direct access to the device's registers. File permissions
prevent random users from accessing the device's registers through these
files, but don't stop a privileged app that chooses to ignore the purpose
of these files from doing so.

There are devices with abnormally strict restrictions with respect to
accessing their registers; aspects that are typically handled by the
device's driver. When these access restrictions are not followed - as
when a userspace app such as "udevadm info --attribute-walk
--path=/sys/..." parses though reading all the device's sysfs entries - it
can cause such devices to fail.

This patch introduces a quirking mechanism that can be used to detect
accesses that do no meet the device's restrictions, letting a device
specific method intervene and decide how to progress.

Reported-by: Xiangliang Yu <yuxiangl@xxxxxxxxxxx>
Signed-off-by: Myron Stowe <myron.stowe@xxxxxxxxxx>

I honestly don't think there's much point in even attempting this strategy. This list of devices in the quirk can't possibly be complete. It would likely be easier to enumerate a white-list of devices that can deal with their IO ports being read willy-nilly than a blacklist of those that don't, as there's likely countless devices that fall into this category. Even if they don't choke as badly as these ones do, it's quite likely that bad behavior will result.

I think there's a few things that need to be done:

-Fix the bug in udevadm that caused it to trawl through these files willy-nilly,

-Fix the kernel so that access through these files complies with the kernel's mechanisms for claiming IO/memory regions to prevent access conflicts (i.e. opening these files should claim the resource region they refer to, and should fail with EBUSY or something if another process or a kernel driver is using it).

-Reconsider whether supporting read/write on the resource files for IO port regions like these makes any sense. Obviously mmap isn't very practical for IO port access on x86 but you could even do something like an ioctl for this purpose. Not very many pieces of software would need to access these files so it's likely OK if the API is a bit ugly. That would prevent something like grepping through sysfs from generating port accesses to random devices.
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