On Tue, Jan 23, 2018 at 05:59:09PM +0530, Arjun Vynipadath wrote:
> Sending on behalf of "Casey Leedom <lee...@chelsio.com>"
> Way back on April 11, 2016 we reported a regression in Linux kernel 4.6-rc2
> brought on by kernel.org commit 104daa71b396. This commit calculates the
> size of a PCI Device's VPD area by parsing the VPD Structure at offset 0x000,
> and restricts accesses to the VPD to that computed size.
> Our devices have a second VPD structure which is located starting at offset
> 0x400 which is the "real" VPD. The 104daa71b396 commit (plus a follow on
> commit 408641e93aa5) caused efforts to read past the end of that computed
> length of the VPD to return silently without error leaving stack junk in the
> VPD read buffers.
> We introduced kernel.org commit cb92148b to allow a driver to tell the
> kernel how large the VPD area really is, introducing a new API
> pci_set_vpd_size() for this purpose.
> Now we've discovered a new subtlety to the problem.
> We have a KVM Hypervisor running a 4.9.70 kernel. So it has all of the
> above commits. When we attach our Physical Function 4 to a Virtual Machine
> and attempt to run cxgb4 in that VM, we see the problem again. The issue is
> that all of the VM Guest OS's efforts to access the PCIe VPD Capability are
> trapped into the KVM 4.9.70 kernel and executed there, with the results
> routed back to the VM Guest OS. The cxgb4 driver in the VM Guest OS uses
> the new pci_set_vpd_size() to notify the OS of the true size of the VPD, but
> that information of course is never sent to the KVM 4.9.70 Hypervisor.
> (And, truth be told, if the Guest OS were older than 4.6, it wouldn't even
> know that it needed to do this.) The result is that again we get silent VPD
> read failures with random stack garbage in the VPD read buffers. (sigh)
Let me pull out one tiny piece of this problem: If the VPD read
returns failure, the caller should not look at the read buffer. But
we should *never* copy random stack garbage into the read buffer, no
matter what the VPD read returns.
I guess it's the 4.9.70 kernel that's putting garbage into the VPD
read buffer? Is this something that needs to be fixed in the current