Re: [PATCH] PCI/bwctrl: Fix NULL pointer deref on bus number exhaustion

["Krzysztof Wilczyński" <kw@linux.com>]

Hello,

> When BIOS neglects to assign bus numbers to PCI bridges, the kernel
> attempts to correct that during PCI device enumeration.  If it runs out
> of bus numbers, no pci_bus is allocated and the "subordinate" pointer in
> the bridge's pci_dev remains NULL.
> 
> The PCIe bandwidth controller erroneously does not check for a NULL
> subordinate pointer and dereferences it on probe.
> 
> Bandwidth control of unusable devices below the bridge is of questionable
> utility, so simply error out instead.  This mirrors what PCIe hotplug does
> since commit 62e4492c3063 ("PCI: Prevent NULL dereference during pciehp
> probe").
> 
> The PCI core emits a message with KERN_INFO severity if it has run out of
> bus numbers.  PCIe hotplug emits an additional message with KERN_ERR
> severity to inform the user that hotplug functionality is disabled at the
> bridge.  A similar message for bandwidth control does not seem merited,
> given that its only purpose so far is to expose an up-to-date link speed
> in sysfs and throttle the link speed on certain laptops with limited
> Thermal Design Power.  So error out silently.
> 
> User-visible messages:
> 
>   pci 0000:16:02.0: bridge configuration invalid ([bus 00-00]), reconfiguring
>   [...]
>   pci_bus 0000:45: busn_res: [bus 45-74] end is updated to 74
>   pci 0000:16:02.0: devices behind bridge are unusable because [bus 45-74] cannot be assigned for them
>   [...]
>   pcieport 0000:16:02.0: pciehp: Hotplug bridge without secondary bus, ignoring
>   [...]
>   BUG: kernel NULL pointer dereference
>   RIP: pcie_update_link_speed
>   pcie_bwnotif_enable
>   pcie_bwnotif_probe
>   pcie_port_probe_service
>   really_probe

Applied to bwctrl, thank you!

	Krzysztof