Linux kernel: powerpc: KVM guest can trigger host crash on Power8

[Michael Ellerman]

The Linux kernel for powerpc since v5.2 has a bug which allows a
malicious KVM guest to crash the host, when the host is running on
Power8.

Only machines using Linux as the hypervisor, aka. KVM, powernv or bare
metal, are affected by the bug. Machines running PowerVM are not
affected.

The bug was introduced in:

    10d91611f426 ("powerpc/64s: Reimplement book3s idle code in C")

Which was first released in v5.2.

The upstream fix is:

  cdeb5d7d890e ("KVM: PPC: Book3S HV: Make idle_kvm_start_guest() return 0 if it went to guest")
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cdeb5d7d890e14f3b70e8087e745c4a6a7d9f337

Which will be included in the v5.16 release.

Note to backporters, the following commits are required:

  73287caa9210ded6066833195f4335f7f688a46b
  ("powerpc64/idle: Fix SP offsets when saving GPRs")

  9b4416c5095c20e110c82ae602c254099b83b72f
  ("KVM: PPC: Book3S HV: Fix stack handling in idle_kvm_start_guest()")

  cdeb5d7d890e14f3b70e8087e745c4a6a7d9f337
  ("KVM: PPC: Book3S HV: Make idle_kvm_start_guest() return 0 if it went to guest")

  496c5fe25c377ddb7815c4ce8ecfb676f051e9b6
  ("powerpc/idle: Don't corrupt back chain when going idle")


I have a test case to trigger the bug, which I can share privately with
anyone who would like to test the fix.

cheers