[PATCH v2 2/3] PCI/PM: Do not save the config space of an inaccessible device

["Francisco Beltrán Millalén" <fbeltranmillalen@gmail.com>]

pci_save_state() reads the standard header into dev->saved_config_space
and marks it valid without checking that the device answered.  If the
device is not accessible, every read returns all ones, the previous
snapshot is overwritten, and pci_restore_state() writes the all-ones
values back once the device answers again.  On a bridge that sets every
writable bit of the Bridge Control register, Secondary Bus Reset
included, and sets the primary, secondary and subordinate bus numbers
to 0xff, which cuts off everything below it.

On a MacBookPro14,3 this happens to the upstream bridge of a
Thunderbolt controller that drops off the bus while the system is
suspending: after resume the bridge answers again, but with bus numbers
ff/ff/ff and Secondary Bus Reset asserted, and the xHCI controllers
behind it are removed.

Commit e18d1abc3bff ("PCI: Avoid saving config space state if
inaccessible") added pci_dev_config_accessible() and checks it before a
reset.  Check it in pci_save_state() itself, so that system suspend,
where the state is saved by pci_pm_suspend_noirq() or by a driver's
suspend_noirq callback, is covered as well.  pci_dev_config_accessible()
reads the Command and Status registers rather than the Vendor and Device
IDs, which always read as all ones on SR-IOV VFs.

Check again after reading, as the device may become inaccessible in the
meantime, and only then replace the previous snapshot of the header and
set state_saved.  The capabilities are saved straight into their own
buffers and are not covered by the second check, and both checks are
racy, as pci_dev_config_accessible() notes.

If the device is not accessible, return -EIO and leave state_saved as it
was.

Assisted-by: LLM
Signed-off-by: Francisco Beltrán Millalén <fbeltranmillalen@gmail.com>
---
v2:
- Use pci_dev_config_accessible() instead of reading the Vendor ID,
  which always reads as all ones on SR-IOV VFs: v1 refused to save the
  state of every VF.  In the v1 thread I said I would use
  pci_device_is_present(), but for a VF that checks the PF and cannot
  tell whether the VF itself answers.
- Do the second check after saving the capabilities, and set
  state_saved only if both checks pass.
- The bus numbers are set to 0xff, not cleared.

 drivers/pci/pci.c |   53 +++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b2879a6be..94de99d31 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1776,31 +1776,52 @@ static void pci_restore_pcix_state(struct pci_dev *dev)
  * pci_save_state - save the PCI configuration space of a device before
  *		    suspending
  * @dev: PCI device that we're dealing with
+ *
+ * If the config space of @dev is not accessible, nothing is saved and the
+ * previous snapshot of the standard header is kept, as writing back the
+ * all-ones values read from such a device would corrupt it once it is
+ * accessible again.
+ *
+ * Return: 0 on success, -EIO if @dev is not accessible, or another
+ * negative errno if a capability could not be saved.
  */
 int pci_save_state(struct pci_dev *dev)
 {
-	int i;
+	u32 config[16];
+	int i, ret;
+
+	if (!pci_dev_config_accessible(dev, "save state"))
+		return -EIO;
+
 	/* XXX: 100% dword access ok here? */
 	for (i = 0; i < 16; i++) {
-		pci_read_config_dword(dev, i * 4, &dev->saved_config_space[i]);
-		pci_dbg(dev, "save config %#04x: %#010x\n",
-			i * 4, dev->saved_config_space[i]);
+		pci_read_config_dword(dev, i * 4, &config[i]);
+		pci_dbg(dev, "save config %#04x: %#010x\n", i * 4, config[i]);
 	}
-	dev->state_saved = true;
 
-	i = pci_save_pcie_state(dev);
-	if (i != 0)
-		return i;
+	ret = pci_save_pcie_state(dev);
+	if (!ret)
+		ret = pci_save_pcix_state(dev);
+	if (!ret) {
+		pci_save_dpc_state(dev);
+		pci_save_aer_state(dev);
+		pci_save_ptm_state(dev);
+		pci_save_tph_state(dev);
+		ret = pci_save_vc_state(dev);
+	}
+
+	/*
+	 * The device may have become inaccessible while it was being read.
+	 * Keep the previous header snapshot in that case.  The capabilities
+	 * are saved directly into their buffers, so they are not protected.
+	 */
+	if (!pci_dev_config_accessible(dev, "save state"))
+		return -EIO;
 
-	i = pci_save_pcix_state(dev);
-	if (i != 0)
-		return i;
+	memcpy(dev->saved_config_space, config, sizeof(config));
+	dev->state_saved = true;
 
-	pci_save_dpc_state(dev);
-	pci_save_aer_state(dev);
-	pci_save_ptm_state(dev);
-	pci_save_tph_state(dev);
-	return pci_save_vc_state(dev);
+	return ret;
 }
 EXPORT_SYMBOL(pci_save_state);