PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free

Struct pcie_link_state->downstream is a pointer to the pci_dev of function
0.  Previously we retained that pointer when removing function 0, and
subsequent ASPM policy changes dereferenced it, resulting in a
use-after-free warning from KASAN, e.g.:

  # echo 1 > /sys/bus/pci/devices/0000:03:00.0/remove
  # echo powersave > /sys/module/pcie_aspm/parameters/policy

  BUG: KASAN: slab-use-after-free in pcie_config_aspm_link+0x42d/0x500
  Call Trace:
   kasan_report+0xae/0xe0
   pcie_config_aspm_link+0x42d/0x500
   pcie_aspm_set_policy+0x8e/0x1a0
   param_attr_store+0x162/0x2c0
   module_attr_store+0x3e/0x80

PCIe spec r6.0, sec 7.5.3.7, recommends that software program the same ASPM
Control value in all functions of multi-function devices.

Disable ASPM and free the pcie_link_state when any child function is
removed so we can discard the dangling pcie_link_state->downstream pointer
and maintain the same ASPM Control configuration for all functions.

[bhelgaas: commit log and comment]
Debugged-by: Zongquan Qin <[email protected]>
Suggested-by: Bjorn Helgaas <[email protected]>
Fixes: b5a0a9b ("PCI/ASPM: Read and set up L1 substate capabilities")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ding Hui <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>

Author: cdkey

drivers/pci/pcie/aspm.c

@@ -1010,29 +1010,32 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)
 
 	down_read(&pci_bus_sem);
 	mutex_lock(&aspm_lock);
-	/*
-	 * All PCIe functions are in one slot, remove one function will remove
-	 * the whole slot, so just wait until we are the last function left.
-	 */
-	if (!list_empty(&parent->subordinate->devices))
-		goto out;
 
 	link = parent->link_state;
 	root = link->root;
 	parent_link = link->parent;
 
-	/* All functions are removed, so just disable ASPM for the link */
+	/*
+	 * 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.
+	 *
+	 * 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.
+	 */
 	pcie_config_aspm_link(link, 0);
 	list_del(&link->sibling);
-	/* Clock PM is for endpoint device */
 	free_link_state(link);
 
 	/* Recheck latencies and configure upstream links */
 	if (parent_link) {
 		pcie_update_aspm_capable(root);
 		pcie_config_aspm_path(parent_link);
 	}
-out:
+
 	mutex_unlock(&aspm_lock);
 	up_read(&pci_bus_sem);
 }