CVE-2021-43056: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-43056 is a vulnerability discovered in the Linux kernel for PowerPC architecture before version 5.14.15. The vulnerability affects the KVM (Kernel-based Virtual Machine) implementation, specifically when running on Power8 processors, due to an implementation bug in arch/powerpc/kvm/book3s_hv_rmhandlers.S handling of the SRR1 register values. The issue was introduced in kernel version 5.2 with commit 10d91611f426 (NVD, Kernel Commit).

The vulnerability stems from an implementation bug in the handling of SRR1 register values in the KVM code. The issue occurs in idle_kvm_start_guest() function, which is called from power7_offline() when a thread has been requested to enter KVM. The function was supposed to return a zero SRR1 value when entering the guest, but this behavior was lost after the code was reworked (Kernel Commit). The CVSS v3.1 base score for this vulnerability is 5.5 (Medium) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (NVD).

When exploited, this vulnerability allows a malicious KVM guest to crash the host system when running on Power8 processors. The issue only affects machines using Linux as the hypervisor (KVM, powernv, or bare metal). Systems running PowerVM are not affected (OSS Security).

The vulnerability was fixed in Linux kernel version 5.14.15. The fix requires applying four patches in sequence: 73287caa9210 (Fix SP offsets when saving GPRs), 9b4416c5095c (Fix stack handling in idle_kvm_start_guest), cdeb5d7d890e (Make idle_kvm_start_guest return 0 if it went to guest), and 496c5fe25c377 (Don't corrupt back chain when going idle) (OSS Security).