CVE-2018-12896 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel through 4.17.3. An Integer Overflow in kernel/time/posix-timers.c in the POSIX timer code is caused by the way the overrun accounting works. Depending on interval and expiry time values, the overrun can be larger than INT_MAX, but the accounting is int based. This basically makes the accounting values, which are visible to user space via timer_getoverrun(2) and siginfo::si_overrun, random. For example, a local user can cause a denial of service (signed integer overflow) via crafted mmap, futex, timer_create, and timer_settime system calls.
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Analysis
by VulDB Data Team • 03/29/2023
The vulnerability described in CVE-2018-12896 represents a critical integer overflow flaw within the Linux kernel's POSIX timer implementation that affects versions through 4.17.3. This issue resides in the kernel/time/posix-timers.c file where the overrun accounting mechanism fails to properly handle large timer values, creating a scenario where the signed integer counter used for tracking timer overruns can exceed the maximum value representable by a 32-bit signed integer. The flaw specifically manifests when interval and expiry time values are manipulated in such a way that the calculated overrun exceeds INT_MAX, which is the maximum value a signed integer can hold in most systems. This overflow condition creates unpredictable behavior in the kernel's timer subsystem and fundamentally compromises the integrity of overrun accounting information that is exposed to user space applications.
The technical execution of this vulnerability involves a sophisticated interplay between multiple kernel system calls that together enable a malicious user to trigger the integer overflow condition. An attacker can exploit this flaw by carefully crafting a sequence of system calls including mmap, futex, timer_create, and timer_settime to manipulate timer parameters in a manner that causes the overrun calculation to exceed the signed integer limit. The overflow occurs during the accounting process where the kernel maintains a count of how many times a timer has overrun its scheduled execution time, and this count is stored in an int variable rather than a larger data type that could accommodate the potentially large values. When the integer overflow occurs, the accounting values become corrupted and essentially random, as the signed integer wraps around to negative values or other unexpected results when the maximum positive value is exceeded.
The operational impact of this vulnerability extends beyond simple data corruption, creating a potential denial of service condition that can be reliably triggered by a local user with minimal privileges. The random nature of the corrupted overrun values makes it difficult for applications to properly handle timer events, leading to unpredictable behavior in timer-based applications that depend on accurate overrun information. Applications that rely on timer_getoverrun(2) system call or examine siginfo::si_overrun fields in signal handlers may experience crashes, incorrect behavior, or denial of service conditions when they encounter these corrupted values. The vulnerability is particularly concerning because it operates at the kernel level where a local user can leverage it to cause system instability or disruption, and the effects can propagate through the entire system as timer-based services and applications respond to the corrupted timer information. This flaw essentially undermines the reliability of POSIX timer functionality and can affect system stability in ways that are difficult to predict or prevent without proper kernel-level fixes.
The root cause of this vulnerability aligns with CWE-190, which identifies integer overflow and underflow conditions as a significant class of software defects that can lead to unpredictable behavior and security consequences. From an attack framework perspective, this vulnerability maps to several ATT&CK techniques including privilege escalation through local exploitation, and system service denial of service attacks. The vulnerability demonstrates the importance of proper integer bounds checking in kernel code, particularly in accounting mechanisms where overflow conditions can have cascading effects on system stability. Mitigation strategies should focus on kernel updates that properly handle timer overruns beyond the signed integer limit, potentially by using larger data types for overflow accounting or implementing proper bounds checking to prevent the overflow condition from occurring. Additionally, system administrators should monitor for applications that depend heavily on timer functionality and ensure proper kernel patching to prevent exploitation of this vulnerability. The fix typically involves modifying the kernel code to use appropriate data types that can handle the full range of possible timer overrun values, preventing the integer overflow that leads to the random accounting values described in the vulnerability.