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

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Message-ID: <1363493482.16793.69.camel@ul30vt.home>
Date:	Sat, 16 Mar 2013 22:11:22 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Myron Stowe <mstowe@...hat.com>,
	Myron Stowe <myron.stowe@...hat.com>, kay@...y.org,
	linux-hotplug@...r.kernel.org, linux-pci@...r.kernel.org,
	yuxiangl@...vell.com, yxlraid@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

On Sat, 2013-03-16 at 18:03 -0700, Greg KH wrote:
> On Sat, Mar 16, 2013 at 05:50:53PM -0600, Myron Stowe wrote:
> > 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
> > https://lkml.org/lkml/2013/3/7/242 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.
> 
> A PCI quirk should handle this properly, right?  Why not do that?  Worse
> thing, the quirk could just not expose these sysfs files for this
> device, which would solve all userspace program issues, right?

Not exactly.  I/O port access through pci-sysfs was added for userspace
programs, specifically qemu-kvm device assignment.  We use the I/O port
resource# files to access device owned I/O port registers using file
permissions rather than global permissions such as iopl/ioperm.  File
permissions also prevent random users from accessing device registers
through these files, but of course can't stop a privileged app that
chooses to ignore the purpose of these files.  A quirk would therefore
remove a file that actually has a useful purpose for one app just so
another app that has no particular reason for dumping the contents can
run unabated.  Thanks,

Alex

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