CVE-2024-58093: 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 parentlink 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 Free exactly when function 0 is removed -- before the parent link is obsolete, but after all subordinate links are gone.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]