[PATCH] kernel/resource.c: invalidate parent when freed resource has childs

[Schmid, Carsten]

When a resource is freed and has children, the childrens are
left without any hint that their parent is no more valid.
This caused at least one use-after-free in the xhci-hcd using
ext-caps driver when platform code released platform devices.

Fix this by setting child's parent to zero and warn.

Signed-off-by: Carsten Schmid <carsten_schmid@xxxxxxxxxx>
---
Rationale:
When hunting for the root cause of a crash on a 4.14.86 kernel, i
have found the root cause and checked it being still present
upstream. Our case:
Having xhci-hcd and intel_xhci_usb_sw active we can see in
/proc/meminfo: (exceirpt)
  b3c00000-b3c0ffff : 0000:00:15.0
    b3c00000-b3c0ffff : xhci-hcd
      b3c08070-b3c0846f : intel_xhci_usb_sw
intel_xhci_usb_sw being a child of xhci-hcd.

Doing an unbind command
echo 0000:00:15.0 > /sys/bus/pci/drivers/xhci_hcd/unbind
leads to xhci-hcd being freed in __release_region.
The intel_xhci_usb_sw resource is accessed in platform code
in platform_device_del with
		for (i = 0; i < pdev->num_resources; i++) {
			struct resource *r = &pdev->resource[i];
			if (r->parent)
				release_resource(r);
		}
as the resource's parent has not been updated, the release_resource
uses the parent:
	p = &old->parent->child;
which is now invalid.
Fix this by marking the parent invalid in the child and give a warning
in dmesg.
---
 kernel/resource.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/resource.c b/kernel/resource.c
index 158f04ec1d4f..95340cb0b1c2 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1200,6 +1200,15 @@ void __release_region(struct resource *parent, resource_size_t start,
 			write_unlock(&resource_lock);
 			if (res->flags & IORESOURCE_MUXED)
 				wake_up(&muxed_resource_wait);
+
+			write_lock(&resource_lock);
+			if (res->child) {
+				printk(KERN_WARNING "__release_region: %s has child %s,"
+						"invalidating childs parent\n",
+						res->name, res->child->name);
+				res->child->parent = NULL;
+			}
+			write_unlock(&resource_lock);
 			free_resource(res);
 			return;
 		}
-- 
2.17.1