CVE-2024-26883 in Linux
Summary
by MITRE • 04/17/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix stackmap overflow check on 32-bit arches
The stackmap code relies on roundup_pow_of_two() to compute the number of hash buckets, and contains an overflow check by checking if the resulting value is 0. However, on 32-bit arches, the roundup code itself can overflow by doing a 32-bit left-shift of an unsigned long value, which is undefined behaviour, so it is not guaranteed to truncate neatly. This was triggered by syzbot on the DEVMAP_HASH type, which contains the same check, copied from the hashtab code.
The commit in the fixes tag actually attempted to fix this, but the fix did not account for the UB, so the fix only works on CPUs where an overflow does result in a neat truncation to zero, which is not guaranteed. Checking the value before rounding does not have this problem.
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Analysis
by VulDB Data Team • 06/20/2025
The vulnerability CVE-2024-26883 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem, specifically affecting the stackmap implementation on 32-bit architectures. This issue demonstrates a critical flaw in memory management and overflow handling that could potentially lead to system instability or security compromise. The vulnerability stems from improper handling of bitwise operations during hash bucket calculation, creating a scenario where undefined behavior can occur during kernel operation. The problem manifests when the stackmap code utilizes the roundup_pow_of_two() function to determine hash bucket counts, a common pattern in hash table implementations that requires careful handling of integer overflow conditions.
The technical flaw occurs in the interaction between the roundup_pow_of_two() function and 32-bit unsigned long integer operations. When processing large input values on 32-bit systems, the function performs a left-shift operation that can overflow the 32-bit unsigned long type, resulting in undefined behavior according to the C standard. This undefined behavior means that the overflow may not consistently truncate to zero as expected, creating a race condition where the overflow check fails to properly detect the problematic state. The vulnerability is particularly concerning because it affects the DEVMAP_HASH type which shares identical code patterns with the original stackmap implementation, indicating a broader systemic issue within the kernel's hash table handling mechanisms.
The operational impact of this vulnerability extends beyond simple memory corruption, potentially enabling attackers to manipulate kernel memory structures through carefully crafted eBPF programs. This flaw creates opportunities for privilege escalation or denial of service attacks, as the undefined behavior during hash bucket calculation could lead to memory corruption that affects kernel stability. The vulnerability specifically impacts 32-bit architectures where the unsigned long type is 32 bits, making it a targeted issue for systems running on older hardware or embedded devices. The original fix attempt failed because it did not address the fundamental undefined behavior issue, relying on implementation-specific truncation behavior that is not guaranteed across different CPU architectures or compiler implementations.
Mitigation strategies for CVE-2024-26883 require immediate kernel updates that implement proper overflow checking before applying the roundup operation, ensuring that the fix works consistently across all architectures. This approach aligns with CWE-191, which addresses integer underflow and overflow conditions, and follows ATT&CK technique T1068, which covers privilege escalation through kernel vulnerabilities. Organizations should prioritize patching affected systems and monitoring for potential exploitation attempts, particularly on 32-bit systems where the vulnerability is most pronounced. The fix should also include comprehensive testing of eBPF program execution to ensure that hash table operations behave correctly under all conditions, preventing the undefined behavior that could lead to system compromise. This vulnerability underscores the importance of careful integer handling in kernel code and demonstrates how seemingly minor implementation details can create significant security risks.