CVE-2021-47255: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, a vulnerability (CVE-2021-47255) was discovered in the KVM LAPIC implementation. The issue relates to illegal APIC register access that could lead to kernel stack content leaks. According to the System Developer's Manual (SDM), any access touching bytes 4 through 15 of an APIC register may cause undefined behavior and must not be executed. This vulnerability was introduced when a guard preventing such access was removed in commit 01402cf81051 ("kvm: LAPIC: write down valid APIC registers") (Kernel Git).

The vulnerability exists in the kvm_lapic_reg_read function where accesses to certain APIC register bytes were not properly restricted. When accessing bytes 4 through 15 of an APIC register, it could result in undefined behavior and potentially leak kernel stack contents. The issue was fixed by restoring the guard that checks if the alignment plus length of the access exceeds 4 bytes, preventing illegal register access (Kernel Git).

The primary impact of this vulnerability is the potential leak of kernel stack contents through improper APIC register access. This could allow an attacker to obtain sensitive information from the kernel memory space (NVD).

The vulnerability can be triggered through KVM's LAPIC register read functionality. It requires access to the KVM virtualization environment to attempt exploitation (NVD).

The vulnerability was fixed by restoring the guard in the kvm_lapic_reg_read function that prevents access to bytes 4 through 15 of APIC registers. The fix involves adding a check 'if (alignment + len > 4) return 1' before allowing register access (Kernel Git).