On Tue, Jan 23, 2018 at 05:59:09PM +0530, Arjun Vynipadath wrote: > Sending on behalf of "Casey Leedom <leedom@xxxxxxxxxxx>" > > 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[1]. 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 upstream kernel?
