CVE-2024-36916 in Linux
Summary
by MITRE • 05/30/2024
In the Linux kernel, the following vulnerability has been resolved:
blk-iocost: avoid out of bounds shift
UBSAN catches undefined behavior in blk-iocost, where sometimes iocg->delay is shifted right by a number that is too large, resulting in undefined behavior on some architectures.
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UBSAN: shift-out-of-bounds in block/blk-iocost.c:1366:23 shift exponent 64 is too large for 64-bit type 'u64' (aka 'unsigned long long') CPU: 16 PID: 0 Comm: swapper/16 Tainted: G S E N 6.9.0-0_fbk700_debug_rc2_kbuilder_0_gc85af715cac0 #1 Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A23 12/08/2020 Call Trace: <IRQ> dump_stack_lvl+0x8f/0xe0 __ubsan_handle_shift_out_of_bounds+0x22c/0x280 iocg_kick_delay+0x30b/0x310 ioc_timer_fn+0x2fb/0x1f80 __run_timer_base+0x1b6/0x250 ...
Avoid that undefined behavior by simply taking the "delay = 0" branch if the shift is too large.
I am not sure what the symptoms of an undefined value delay will be, but I suspect it could be more than a little annoying to debug.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 01/22/2026
The vulnerability described in CVE-2024-36916 resides within the Linux kernel's block I/O control subsystem, specifically in the blk-iocost component responsible for managing I/O cost accounting. This component plays a critical role in Quality of Service (QoS) management for storage devices by tracking and controlling I/O operations based on cost metrics. The issue manifests as an undefined behavior condition that occurs during right-shift operations on 64-bit unsigned integers, where the shift exponent exceeds the valid range for the data type. The undefined behavior is detected by the Undefined Behavior Sanitizer (UBSAN) which is part of the kernel's debugging infrastructure designed to catch programming errors that could lead to security vulnerabilities or system instability.
The technical flaw occurs in the block/blk-iocost.c file at line 1366 within the iocg_kick_delay function where the code attempts to perform a right-shift operation on the iocg->delay variable. When the shift exponent reaches 64 bits for a 64-bit unsigned long long type, the operation becomes undefined according to the C standard, as shifting by the full width of the data type or more results in undefined behavior across most architectures. This particular scenario represents a classic case of CWE-758: Reliance on Undefined Behavior, where the code's execution path leads to implementation-defined or undefined behavior that can cause unpredictable system states. The UBSAN error message specifically indicates that a shift exponent of 64 is being applied to a u64 type, which is outside the valid range for such operations.
The operational impact of this vulnerability extends beyond simple debugging complications as undefined behavior in kernel space can lead to unpredictable system states including potential system crashes, data corruption, or even privilege escalation opportunities. The vulnerability's potential security implications stem from the fact that kernel-level undefined behavior can be exploited by malicious actors to gain unauthorized access or cause denial of service conditions. While the immediate symptoms might appear benign, such as system instability or occasional crashes, the underlying issue creates a potential attack surface that could be leveraged to compromise system integrity. The vulnerability's presence in the I/O cost management subsystem means that any I/O operations involving delayed processing could trigger the undefined behavior, making it particularly concerning for systems under heavy I/O load or those using advanced storage configurations.
Mitigation strategies for this vulnerability involve implementing defensive programming practices that prevent the occurrence of undefined behavior in kernel code. The recommended fix involves adding a conditional check to ensure that the shift exponent does not exceed the valid range for the data type before performing the shift operation. This approach aligns with the principle of input validation and defensive programming commonly referenced in security best practices. The solution specifically involves taking the delay = 0 branch when the shift operation would result in an invalid shift exponent, effectively preventing the undefined behavior while maintaining the intended functionality of the I/O cost management system. Organizations should prioritize applying the kernel patch that implements this fix to prevent exploitation, as the vulnerability could potentially be leveraged to create persistent system instability or serve as a stepping stone for more sophisticated attacks. System administrators should also consider monitoring for UBSAN warnings and kernel crash reports that might indicate the presence of this vulnerability in production environments, particularly in systems that rely heavily on I/O operations and storage virtualization.