Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

From: Myron Stowe
Date: Sat Mar 16 2013 - 19:51:23 EST

On Sat, 2013-03-16 at 15:11 -0700, Greg KH wrote:
> On Sat, Mar 16, 2013 at 03:35:19PM -0600, Myron Stowe wrote:
> > Sysfs includes entries to memory that backs a PCI device's BARs, both I/O
> > Port space and MMIO. This memory regions correspond to the device's
> > internal status and control registers used to drive the device.
> >
> > Accessing these registers from userspace such as "udevadm info
> > --attribute-walk --path=/sys/devices/..." does can not be allowed as
> > such accesses outside of the driver, even just reading, can yield
> > catastrophic consequences.
> >
> > Udevadm-info skips parsing a specific set of sysfs entries including
> > 'resource'. This patch extends the set to include the additional
> > 'resource<N>' entries that correspond to a PCI device's BARs.
> Nice, are you also going to patch bash to prevent a user from reading
> these sysfs files as well? :)
> And pciutils?
> You get my point here, right? The root user just asked to read all of
> the data for this device, so why wouldn't you allow it? Just like
> 'lspci' does. Or bash does.

Yes :P , you raise a very good point, there are a lot of way a user can
poke around in those BARs. However, there is a difference between
shooting yourself in the foot and getting what you deserve versus
unknowingly executing a common command such as udevadm and having the
system hang.
> If this hardware has a problem, then it needs to be fixed in the kernel,
> not have random band-aids added to various userspace programs to paper
> over the root problem here. Please fix the kernel driver and all should
> be fine. No need to change udevadm.

Xiangliang initially proposed a patch within the PCI core. Ignoring the
specific issue with the proposal which I pointed out in the thread, that just doesn't seem like
the right place to effect a change either as PCI's core isn't concerned
with the contents or access limitations of those regions, those are
issues that the driver concerns itself with.

So things seem to be gravitating towards the driver. I'm fairly
ignorant of this area but as Robert succinctly pointed out in the
originating thread - the AHCI driver only uses the device's MMIO region.
The I/O related regions are for legacy SFF-compatible ATA ports and are
not used to driver the device. This, coupled with the observance that
userspace accesses such as udevadm, and others like you additionally
point out, do not filter through the device's driver for seems to
suggest that changes to the driver will not help here either.

That said, I was attempting to point out an interesting problem and get
the conversation started towards coming up with some type a solution.
Let's continue the conversation and see where things go.

> greg k-h

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