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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Bjorn Helgaas <bhelgaas@google.com>
Subject: [PATCH 4.9 24/65] PCI/ASPM: Deal with missing root ports in link state handling
Date:   Fri,  9 Mar 2018 16:18:24 -0800
Message-Id: <20180310001826.826066317@linuxfoundation.org>
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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>

commit ee8bdfb6568d86bb93f55f8d99c4c643e77304ee upstream.

Even though it is unconventional, some PCIe host implementations omit the
root ports entirely, and simply consist of a host bridge (which is not
modeled as a device in the PCI hierarchy) and a link.

When the downstream device is an endpoint, our current code does not seem
to mind this unusual configuration. However, when PCIe switches are
involved, the ASPM code assumes that any downstream switch port has a
parent, and blindly dereferences the bus->parent->self field of the pci_dev
struct to chain the downstream link state to the link state of the root
port. Given that the root port is missing, the link is not modeled at all,
and nor is the link state, and attempting to access it results in a NULL
pointer dereference and a crash.

Avoid this by allowing the link state chain to terminate at the downstream
port if no root port exists.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/pci/pcie/aspm.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -526,10 +526,14 @@ static struct pcie_link_state *alloc_pci
 
 	/*
 	 * Root Ports and PCI/PCI-X to PCIe Bridges are roots of PCIe
-	 * hierarchies.
+	 * hierarchies.  Note that some PCIe host implementations omit
+	 * the root ports entirely, in which case a downstream port on
+	 * a switch may become the root of the link state chain for all
+	 * its subordinate endpoints.
 	 */
 	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_ROOT_PORT ||
-	    pci_pcie_type(pdev) == PCI_EXP_TYPE_PCIE_BRIDGE) {
+	    pci_pcie_type(pdev) == PCI_EXP_TYPE_PCIE_BRIDGE ||
+	    !pdev->bus->parent->self) {
 		link->root = link;
 	} else {
 		struct pcie_link_state *parent;