CVE-2022-49955

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

powerpc/rtas: Fix RTAS MSR[HV] handling for Cell

The semi-recent changes to MSR handling when entering RTAS (firmware)
cause crashes on IBM Cell machines. An example trace:

  kernel tried to execute user page (2fff01a8) - exploit attempt? (uid: 0)
  BUG: Unable to handle kernel instruction fetch
  Faulting instruction address: 0x2fff01a8
  Oops: Kernel access of bad area, sig: 11 [#1]
  BE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=4 NUMA Cell
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W          6.0.0-rc2-00433-gede0a8d3307a #207
  NIP:  000000002fff01a8 LR: 0000000000032608 CTR: 0000000000000000
  REGS: c0000000015236b0 TRAP: 0400   Tainted: G        W           (6.0.0-rc2-00433-gede0a8d3307a)
  MSR:  0000000008001002 <ME,RI>  CR: 00000000  XER: 20000000
  ...
  NIP 0x2fff01a8
  LR  0x32608
  Call Trace:
    0xc00000000143c5f8 (unreliable)
    .rtas_call+0x224/0x320
    .rtas_get_boot_time+0x70/0x150
    .read_persistent_clock64+0x114/0x140
    .read_persistent_wall_and_boot_offset+0x24/0x80
    .timekeeping_init+0x40/0x29c
    .start_kernel+0x674/0x8f0
    start_here_common+0x1c/0x50

Unlike PAPR platforms where RTAS is only used in guests, on the IBM Cell
machines Linux runs with MSR[HV] set but also uses RTAS, provided by
SLOF.

Fix it by copying the MSR[HV] bit from the MSR value we've just read
using mfmsr into the value used for RTAS.

It seems like we could also fix it using an #ifdef CELL to set MSR[HV],
but that doesn't work because it's possible to build a single kernel
image that runs on both Cell native and pseries.