CVE-2022-49955: 
Linux Kernel vulnerability analysis and mitigation

CVE-2022-49955 affects the Linux kernel's powerpc/rtas component, specifically related to MSR[HV] handling for IBM Cell machines. The vulnerability was discovered when recent changes to MSR handling when entering RTAS (firmware) caused crashes on IBM Cell machines (Kernel NVD).

The vulnerability manifests when the system attempts to execute user page instructions, resulting in kernel instruction fetch failures and crashes. The issue occurs because unlike PAPR platforms where RTAS is only used in guests, on IBM Cell machines Linux runs with MSR[HV] set but also uses RTAS provided by SLOF. The bug is triggered when attempting to access instruction address 0x2fff01a8, leading to kernel access of bad area and system crashes (Kernel NVD).

When exploited, the vulnerability causes system crashes on affected IBM Cell machines, resulting in denial of service conditions. The system generates a BUG report indicating 'Unable to handle kernel instruction fetch' and 'Kernel access of bad area' (Kernel NVD).

The fix involves copying the MSR[HV] bit from the MSR value read using mfmsr into the value used for RTAS. This solution was chosen over using an #ifdef CELL to set MSR[HV] because it's possible to build a single kernel image that runs on both Cell native and pseries (Kernel NVD).