CVE-2022-49955: NULL Pointer Dereference

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.