CVE-2024-50254 in Linux
Summary
by MITRE • 11/09/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Free dynamically allocated bits in bpf_iter_bits_destroy()
bpf_iter_bits_destroy() uses "kit->nr_bits <= 64" to check whether the bits are dynamically allocated. However, the check is incorrect and may cause a kmemleak as shown below:
unreferenced object 0xffff88812628c8c0 (size 32): comm "swapper/0", pid 1, jiffies 4294727320 hex dump (first 32 bytes): b0 c1 55 f5 81 88 ff ff f0 f0 f0 f0 f0 f0 f0 f0 ..U........... f0 f0 f0 f0 f0 f0 f0 f0 00 00 00 00 00 00 00 00 .............. backtrace (crc 781e32cc): [] kmemleak_alloc+0x4b/0x80
[] __kmalloc_node_noprof+0x480/0x5c0
[] __alloc.isra.0+0x89/0xb0
[] alloc_bulk+0x2af/0x720
[] prefill_mem_cache+0x7f/0xb0
[] bpf_mem_alloc_init+0x3e2/0x610
[] bpf_global_ma_init+0x19/0x30
[] do_one_initcall+0xd3/0x3c0
[] kernel_init_freeable+0x66a/0x940
[] kernel_init+0x20/0x160
[] ret_from_fork+0x3c/0x70
[] ret_from_fork_asm+0x1a/0x30
That is because nr_bits will be set as zero in bpf_iter_bits_next() after all bits have been iterated.
Fix the issue by setting kit->bit to kit->nr_bits instead of setting kit->nr_bits to zero when the iteration completes in bpf_iter_bits_next(). In addition, use "!nr_bits || bits >= nr_bits" to check whether the iteration is complete and still use "nr_bits > 64" to indicate whether bits are dynamically allocated. The "!nr_bits" check is necessary because bpf_iter_bits_new() may fail before setting kit->nr_bits, and this condition will stop the iteration early instead of accessing the zeroed or freed kit->bits.
Considering the initial value of kit->bits is -1 and the type of kit->nr_bits is unsigned int, change the type of kit->nr_bits to int. The potential overflow problem will be handled in the following patch.
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Analysis
by VulDB Data Team • 10/02/2025
The vulnerability identified as CVE-2024-50254 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem, specifically in the bpf_iter_bits_destroy() function. This flaw represents a memory management issue that could lead to kmemleak conditions and potential information disclosure or system instability. The vulnerability is classified under CWE-401 as a failure to release memory, which directly impacts the kernel's memory management integrity. The issue stems from incorrect logic in determining when dynamically allocated memory should be freed, creating a scenario where memory allocated for bit iteration operations persists in memory even after it should have been released.
The technical flaw manifests in the bpf_iter_bits_next() function where the iteration completion check is flawed. The original code used kit->nr_bits <= 64 to determine if bits were dynamically allocated, but this condition fails to properly handle the case where all bits have been iterated and nr_bits is set to zero. This incorrect check causes the system to maintain references to memory that should have been freed, resulting in memory leaks that can accumulate over time and potentially impact system performance or stability. The kernel's memory leak detection mechanism (kmemleak) identifies these unreferenced objects, as demonstrated in the provided stack trace showing the memory allocation path through bpf_mem_alloc_init and bpf_global_ma_init functions. The improper handling of the iteration state creates a condition where freed memory may be accessed or where the memory management system fails to properly track allocated resources.
The operational impact of this vulnerability extends beyond simple memory leaks, potentially affecting system reliability and resource utilization in environments heavily reliant on eBPF operations. Attackers could exploit this condition to cause resource exhaustion through repeated memory allocations that are never properly freed, or in more sophisticated scenarios, potentially leverage the leaked memory for information disclosure attacks. The vulnerability affects systems running Linux kernels with eBPF support, particularly those utilizing bpf_iter_bits operations for iterating through bit arrays. The memory leak occurs during kernel initialization and early boot processes, making it particularly concerning for systems that rely on eBPF for network filtering, tracing, or monitoring operations. This vulnerability aligns with ATT&CK technique T1059.006 for kernel-mode rootkits and T1490 for resource hijacking, as it represents a mechanism for maintaining persistent memory references that could be exploited for further attacks.
The fix implemented addresses the core issue by modifying the bpf_iter_bits_next() function to properly handle iteration completion. The solution changes the logic to set kit->bit to kit->nr_bits instead of zeroing kit->nr_bits, ensuring proper tracking of iteration state. Additionally, the condition check is updated to use "!nr_bits || bits >= nr_bits" which properly handles early termination scenarios where bpf_iter_bits_new() may fail before setting kit->nr_bits. The fix also includes a type change from unsigned int to int for kit->nr_bits to prevent potential overflow issues, though the overflow handling is noted as being addressed in a subsequent patch. This remediation directly addresses the memory management error by ensuring that dynamically allocated memory is properly freed when the iteration completes, preventing the kmemleak conditions that were previously occurring. The solution maintains backward compatibility while fixing the core memory management flaw, ensuring that the eBPF subsystem operates correctly without leaving memory in an unreferenced state. The patch demonstrates proper kernel memory management practices that align with security standards for kernel code and helps prevent potential escalation to more serious security vulnerabilities through memory corruption or resource exhaustion attacks.