CVE-2024-42302: PCI/DPC: Fix use-after-free on concurrent DPC and hot-removal

In the Linux kernel, the following vulnerability has been resolved:

PCI/DPC: Fix use-after-free on concurrent DPC and hot-removal

Keith reports a use-after-free when a DPC event occurs concurrently to hot-removal of the same portion of the hierarchy:

The dpchandler() awaits readiness of the secondary bus below the Downstream Port where the DPC event occurred. To do so, it polls the config space of the first child device on the secondary bus. If that child device is concurrently removed, accesses to its struct pcidev cause the kernel to oops.

That's because pcibridgewaitforsecondarybus() neglects to hold a reference on the child device. Before v6.3, the function was only called on resume from system sleep or on runtime resume. Holding a reference wasn't necessary back then because the pciehp IRQ thread could never run concurrently. (On resume from system sleep, IRQs are not enabled until after the resumenoirq phase. And runtime resume is always awaited before a PCI device is removed.)

However starting with v6.3, pcibridgewaitforsecondarybus() is also called on a DPC event. Commit 53b54ad074de ("PCI/DPC: Await readiness of secondary bus after reset"), which introduced that, failed to appreciate that pcibridgewaitforsecondarybus() now needs to hold a reference on the child device because dpchandler() and pciehp may indeed run concurrently. The commit was backported to v5.10+ stable kernels, so that's the oldest one affected.

Add the missing reference acquisition.

Abridged stack trace:

BUG: unable to handle page fault for address: 00000000091400c0 CPU: 15 PID: 2464 Comm: irq/53-pcie-dpc 6.9.0 RIP: pcibusreadconfigdword+0x17/0x50 pcidevwait() pcibridgewaitforsecondarybus() dpcresetlink() pciedorecovery() dpchandler()