PCI/ASPM: Fix link state exit during switch upstream function removal

[ Upstream commit cbf937d ]

Before 456d8aa ("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Free exactly when function 0 is removed -- before the parent link is
obsolete, but after all subordinate links are gone.

Link: https://lore.kernel.org/r/e12898835f25234561c9d7de4435590d957b85d9.1734924854.git.dns@arista.com
Fixes: 456d8aa ("PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free")
Signed-off-by: Daniel Stodden <dns@arista.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/pci/pcie/aspm.c

@@ -977,16 +977,16 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)
 	parent_link = link->parent;
 
 	/*
-	 * link->downstream is a pointer to the pci_dev of function 0.  If
-	 * we remove that function, the pci_dev is about to be deallocated,
-	 * so we can't use link->downstream again.  Free the link state to
-	 * avoid this.
+	 * Free the parent link state, no later than function 0 (i.e.
+	 * link->downstream) being removed.
 	 *
-	 * If we're removing a non-0 function, it's possible we could
-	 * retain the link state, but PCIe r6.0, sec 7.5.3.7, recommends
-	 * programming the same ASPM Control value for all functions of
-	 * multi-function devices, so disable ASPM for all of them.
+	 * Do not free the link state any earlier. If function 0 is a
+	 * switch upstream port, this link state is parent_link to all
+	 * subordinate ones.
 	 */
+	if (pdev != link->downstream)
+		goto out;
+
 	pcie_config_aspm_link(link, 0);
 	list_del(&link->sibling);
 	free_link_state(link);
@@ -997,6 +997,7 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)
 		pcie_config_aspm_path(parent_link);
 	}
 
+ out:
 	mutex_unlock(&aspm_lock);
 	up_read(&pci_bus_sem);
 }