On Sun, 2013-03-17 at 08:33 -0600, Myron Stowe wrote: > On Sun, 2013-03-17 at 07:38 -0600, Alex Williamson wrote: > > On Sat, 2013-03-16 at 22:36 -0700, Greg KH wrote: > > > On Sat, Mar 16, 2013 at 10:11:22PM -0600, Alex Williamson wrote: > > > > 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, > > > > > > The quirk would only be for this one specific device, which obviously > > > can't handle this type of access, so why would you want the sysfs files > > > even present for it at all? > > > > I'm assuming that the device only breaks because udevadm is dumping the > > full I/O port register space of the device and that if an actual driver > > was interacting with it through this interface that it would work. > > Correct: > 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 Greg additionally pointed out, do not filter through the > device's driver seems to suggest that changes to the driver will > not help here either. That may be true of our AHCI driver, but when it's assigned to a guest we're potentially using a completely different stack and cannot make that assumption. A guest running in compatibility mode or the option ROM for the device may still use I/O port regions. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
