CVE-2024-40904 in Linux
Summary
by MITRE • 07/12/2024
In the Linux kernel, the following vulnerability has been resolved:
USB: class: cdc-wdm: Fix CPU lockup caused by excessive log messages
The syzbot fuzzer found that the interrupt-URB completion callback in the cdc-wdm driver was taking too long, and the driver's immediate resubmission of interrupt URBs with -EPROTO status combined with the dummy-hcd emulation to cause a CPU lockup:
cdc_wdm 1-1:1.0: nonzero urb status received: -71 cdc_wdm 1-1:1.0: wdm_int_callback - 0 bytes watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [syz-executor782:6625]
CPU#0 Utilization every 4s during lockup: #1: 98% system, 0% softirq, 3% hardirq, 0% idle #2: 98% system, 0% softirq, 3% hardirq, 0% idle #3: 98% system, 0% softirq, 3% hardirq, 0% idle #4: 98% system, 0% softirq, 3% hardirq, 0% idle #5: 98% system, 1% softirq, 3% hardirq, 0% idle Modules linked in: irq event stamp: 73096 hardirqs last enabled at (73095): [] console_emit_next_record kernel/printk/printk.c:2935 [inline]
hardirqs last enabled at (73095): [] console_flush_all+0x650/0xb74 kernel/printk/printk.c:2994
hardirqs last disabled at (73096): [] __el1_irq arch/arm64/kernel/entry-common.c:533 [inline]
hardirqs last disabled at (73096): [] el1_interrupt+0x24/0x68 arch/arm64/kernel/entry-common.c:551
softirqs last enabled at (73048): [] softirq_handle_end kernel/softirq.c:400 [inline]
softirqs last enabled at (73048): [] handle_softirqs+0xa60/0xc34 kernel/softirq.c:582
softirqs last disabled at (73043): [] __do_softirq+0x14/0x20 kernel/softirq.c:588
CPU: 0 PID: 6625 Comm: syz-executor782 Tainted: G W 6.10.0-rc2-syzkaller-g8867bbd4a056 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
Testing showed that the problem did not occur if the two error messages -- the first two lines above -- were removed; apparently adding material to the kernel log takes a surprisingly large amount of time.
In any case, the best approach for preventing these lockups and to avoid spamming the log with thousands of error messages per second is to ratelimit the two dev_err() calls. Therefore we replace them with dev_err_ratelimited().
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Analysis
by VulDB Data Team • 09/27/2024
The vulnerability identified as CVE-2024-40904 affects the Linux kernel's USB communication subsystem, specifically within the cdc-wdm driver component. This driver handles communication with USB devices that implement the CDC WDM (USB Communication Device Class - Wireless Device Management) interface, commonly used in cellular modems and similar USB communication devices. The issue manifests as a potential system lockup that can occur during USB device enumeration or operation, particularly when the driver encounters error conditions in interrupt-URB (USB Request Block) completions.
The root cause of this vulnerability lies in the interrupt-URB completion callback function within the cdc-wdm driver. When the driver receives a URB with a -EPROTO status (indicating a protocol error), it immediately resubmits the interrupt URB while simultaneously generating multiple error log messages. This combination creates a feedback loop where the driver continuously processes errors and generates log output, leading to excessive CPU utilization and ultimately causing a soft lockup condition. The syzbot fuzzer identified this pattern during automated testing, demonstrating that the system becomes unresponsive as the CPU becomes saturated with error message generation and URB processing activities.
The operational impact of this vulnerability is significant, as it can cause complete system lockups that prevent normal operation of the affected Linux system. The lockup occurs because the kernel's logging mechanism becomes overwhelmed with error messages, consuming substantial CPU cycles that should be available for normal system operations. This behavior is particularly concerning in embedded systems, servers, or devices where system stability is critical, as a single problematic USB device can bring down an entire system. The vulnerability demonstrates how seemingly minor logging issues can cascade into major system stability problems, especially when combined with driver behavior that creates continuous error conditions.
The mitigation implemented for this vulnerability involves replacing the standard dev_err() logging calls with dev_err_ratelimited() function calls. This change implements rate limiting for error messages, preventing the driver from flooding the kernel log with thousands of identical error messages per second. The rate limiting ensures that while errors are still logged, they occur at a controlled frequency that does not overwhelm system resources. This approach directly addresses the CWE-704 weakness category related to improper handling of error conditions and aligns with ATT&CK technique T1490 for resource exhaustion through logging, preventing the malicious or accidental exploitation of logging mechanisms to cause system instability. The fix demonstrates proper defensive programming practices by ensuring that error reporting does not become a performance bottleneck or system stability threat.
The vulnerability highlights important security considerations in kernel driver development, particularly around the balance between diagnostic information and system stability. It shows how error handling mechanisms that appear benign can become serious threats when combined with specific system conditions. The use of rate limiting in kernel drivers represents a best practice for preventing resource exhaustion attacks and ensuring that diagnostic mechanisms do not become attack vectors themselves. This fix also emphasizes the importance of thorough testing with fuzzing tools like syzbot, which can identify edge cases that might not be apparent through normal testing procedures, particularly in complex subsystem interactions like USB device handling and interrupt processing.