PCI/ASPM: Fix link state exit during switch upstream function removal
In the Linux kernel, the following vulnerability has been resolved:
PCI/ASPM: Fix link state exit during switch upstream function removal
Before 456d8aa37d0f ("PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free"), we would free the ASPM link only after the last function on the bus pertaining to the given link was removed.
That was too late. If function 0 is removed before sibling function, link->downstream would point to free'd memory after.
After above change, we freed the ASPM parent link state upon any function removal on the bus pertaining to a given link.
That is too early. If the link is to a PCIe switch with MFD on the upstream port, then removing functions other than 0 first would free a link which still remains parent_link to the remaining downstream ports.
The resulting GPFs are especially frequent during hot-unplug, because pciehp removes devices on the link bus in reverse order.
On that switch, function 0 is the virtual P2P bridge to the internal bus. Free exactly when function 0 is removed -- before the parent link is obsolete, but after all subordinate links are gone.
[kwilczynski: commit log]