Linux Kernel PowerPC Kexec SMT Re-enablement Vulnerability

A vulnerability exists in the Linux kernel's handling of simultaneous multithreading (SMT) for PowerPC architectures during the kexec process. When SMT is turned off or only partially active, the system fails to properly wake offline CPUs after a new kernel image is loaded via kexec. This issue arises because the add_cpu() function cannot complete its task, as it encounters CPUs that are not considered bootable under the current SMT settings. The problem is addressed by re-enabling SMT and adjusting the SMT thread count to match the number of threads per core before all present CPUs are activated.

Impact

Failing to properly manage CPU states can lead to unreliable system behavior during kernel reboots, potentially causing missed processes or tasks that should be handled by the CPU.

Reproduction

To reproduce this vulnerability, load a new kernel image using the kexec command while SMT is disabled or in a partial state. Then, observe the system's attempt to wake offline CPUs, which will trigger a warning about the CPU being offline. This warning indicates that the kexec process is not able to properly manage CPU states due to the SMT configuration.

Remediation

The vulnerability has been addressed in Linux kernel versions 6.6 and later. Users should upgrade to these versions to ensure proper SMT management during the kexec process.