CVE-2020-27171: 
Linux Kernel vulnerability analysis and mitigation

CVE-2020-27171 is a vulnerability discovered in the Linux kernel before version 5.11.8. The issue exists in kernel/bpf/verifier.c where an off-by-one error (resulting in an integer underflow) affects out-of-bounds speculation on pointer arithmetic, leading to side-channel attacks that can defeat Spectre mitigations and obtain sensitive information from kernel memory (NVD, OSS Security).

The vulnerability stems from a numeric error in the Linux kernel's mechanism to mitigate speculatively out-of-bounds loads. The issue occurs when computing ptr_limit for preventing out-of-bounds speculation on pointer arithmetic. The computation of ptr_limit is off-by-one whenever the pointer moves to the left. When subtracting zero offset from a pointer already at the beginning of a map element value, the computed ptr_limit becomes zero, leading to an integer underflow in fixup_bpf_calls() where sanitization code is generated. This results in loading 0xffffffff into AX instead of sign-extending to the full 64-bit range, allowing abuse for executing speculatively out-of-bounds loads against a 4GB window of address space (Kernel Patch).

The vulnerability allows unprivileged local users to execute speculatively out-of-bounds loads from a 4GB window within kernel memory. Through side-channel attacks, attackers can extract sensitive information from kernel memory, effectively bypassing Spectre mitigations (OSS Security).

The vulnerability can be exploited by unprivileged BPF programs running on affected 64-bit systems. A proof-of-concept exploit was developed by the researcher and shared privately to assist with fix development. The vulnerability has been successfully reproduced against Linux kernel v5.12-rc3 using specific BPF program logic attached to a socket (OSS Security).

The issue was fixed in Linux kernel version 5.11.8. The fix includes a patch to correct the off-by-one error in creating mask to left, along with additional hardening measures. The complete fix series includes multiple patches addressing both this vulnerability and related issues, including prohibiting ALU operations for pointer types not defining ptr_limit and adding sanity checks for upper ptr_limit (Kernel Patch).