CVE-2023-53024: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2023-53024

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix pointer-leak due to insufficient speculative store bypass mitigation To mitigate Spectre v4, 2039f26f3aca ("bpf: Fix leakage due to insufficient speculative store bypass mitigation") inserts lfence instructions after 1) initializing a stack slot and 2) spilling a pointer to the stack. However, this does not cover cases where a stack slot is first initialized with a pointer (subject to sanitization) but then overwritten with a scalar (not subject to sanitization because the slot was already initialized). In this case, the second write may be subject to speculative store bypass (SSB) creating a speculative pointer-as-scalar type confusion. This allows the program to subsequently leak the numerical pointer value using, for example, a branch-based cache side channel. To fix this, also sanitize scalars if they write a stack slot that previously contained a pointer. Assuming that pointer-spills are only generated by LLVM on register-pressure, the performance impact on most real-world BPF programs should be small. The following unprivileged BPF bytecode drafts a minimal exploit and the mitigation: [...] // r6 = 0 or 1 (skalar, unknown user input) // r7 = accessible ptr for side channel // r10 = frame pointer (fp), to be leaked // r9 = r10 # fp alias to encourage ssb *(u64 *)(r9 - 8) = r10 // fp[-8] = ptr, to be leaked // lfence added here because of pointer spill to stack. // // Ommitted: Dummy bpf_ringbuf_output() here to train alias predictor // for no r9-r10 dependency. // *(u64 *)(r10 - 8) = r6 // fp[-8] = scalar, overwrites ptr // 2039f26f3aca: no lfence added because stack slot was not STACK_INVALID, // store may be subject to SSB // // fix: also add an lfence when the slot contained a ptr // r8 = *(u64 *)(r9 - 8) // r8 = architecturally a scalar, speculatively a ptr // // leak ptr using branch-based cache side channel: r8 &= 1 // choose bit to leak if r8 == 0 goto SLOW // no mispredict // architecturally dead code if input r6 is 0, // only executes speculatively iff ptr bit is 1 r8 = *(u64 *)(r7 + 0) # encode bit in cache (0: slow, 1: fast) SLOW: [...] After running this, the program can time the access to *(r7 + 0) to determine whether the chosen pointer bit was 0 or 1. Repeat this 64 times to recover the whole address on amd64. In summary, sanitization can only be skipped if one scalar is overwritten with another scalar. Scalar-confusion due to speculative store bypass can not lead to invalid accesses because the pointer bounds deducted during verification are enforced using branchless logic. See 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic") for details. Do not make the mitigation depend on !env->allow_{uninit_stack,ptr_leaks} because speculative leaks are likely unexpected if these were enabled. For example, leaking the address to a protected log file may be acceptable while disabling the mitigation might unintentionally leak the address into the cached-state of a map that is accessible to unprivileged processes.

AI Analysis

Technical Summary

CVE-2023-53024 is a vulnerability in the Linux kernel's Berkeley Packet Filter (BPF) subsystem related to speculative execution side-channel attacks, specifically a pointer-leak caused by insufficient mitigation of speculative store bypass (SSB), also known as Spectre variant 4. The vulnerability arises because the existing mitigation inserted lfence instructions only after initializing stack slots or spilling pointers to the stack, but failed to sanitize cases where a stack slot initially containing a pointer is overwritten by a scalar value. This creates a speculative pointer-as-scalar type confusion during speculative execution, allowing an attacker to leak pointer values via side channels such as branch-based cache timing attacks. The exploit involves carefully crafted unprivileged BPF bytecode that manipulates stack slots and uses speculative execution to leak kernel pointer addresses. The vulnerability is subtle because it exploits speculative execution behavior rather than direct memory corruption, and it bypasses prior mitigations that assumed scalars overwriting scalars were safe. The fix involves extending sanitization to scalars that overwrite stack slots previously containing pointers, ensuring lfence instructions prevent speculative leaks in these cases. This fix is expected to have minimal performance impact on real-world BPF programs. The vulnerability affects multiple Linux kernel versions prior to the patch commit 2039f26f3aca and does not require privileged access or user interaction to exploit, but does require the ability to load and run unprivileged BPF programs. Although no known exploits are currently in the wild, the vulnerability could allow attackers to leak sensitive kernel memory addresses, potentially aiding further exploitation such as bypassing kernel address space layout randomization (KASLR).

Potential Impact

For European organizations, this vulnerability poses a significant risk primarily to systems running vulnerable Linux kernel versions with unprivileged BPF enabled, which is common in many server, cloud, and container environments. Successful exploitation could allow attackers to leak kernel memory addresses, undermining kernel memory protections and facilitating privilege escalation or other advanced attacks. This is especially critical for organizations relying on Linux-based infrastructure for sensitive workloads, including financial institutions, government agencies, and critical infrastructure operators. The ability to run unprivileged BPF programs is widespread in modern Linux distributions, increasing the attack surface. Although the vulnerability does not directly allow code execution or denial of service, the information disclosure can be a stepping stone for more severe attacks. Given the prevalence of Linux in European data centers, cloud providers, and embedded systems, the vulnerability could impact confidentiality and integrity of sensitive data and systems if exploited. Additionally, the subtlety of the attack and its reliance on speculative execution side channels make detection and mitigation challenging without applying the patch.

Mitigation Recommendations

European organizations should prioritize updating their Linux kernels to versions that include the patch fixing CVE-2023-53024. Specifically, kernel versions incorporating commit 2039f26f3aca or later should be deployed. For environments where immediate patching is not feasible, organizations should consider disabling unprivileged BPF program loading via kernel configuration or runtime parameters (e.g., setting /proc/sys/kernel/unprivileged_bpf_disabled=1) to reduce the attack surface. Monitoring for unusual BPF program loading or execution activity can help detect exploitation attempts. Additionally, organizations should ensure that Spectre and other speculative execution mitigations are enabled and up to date on their systems. Containerized environments should be reviewed to restrict capabilities that allow unprivileged BPF usage. Security teams should also audit kernel configurations related to speculative execution mitigations and uninitialized stack or pointer leak allowances, as the vulnerability mitigation depends on these settings. Finally, applying defense-in-depth controls such as kernel lockdown modes, mandatory access controls (e.g., SELinux, AppArmor), and limiting user privileges can reduce the risk of exploitation.