CVE-2026-89765 Overview
CVE-2026-89765 is an information disclosure vulnerability in the Linux kernel's interval timer (itimer) subsystem. The flaw resides in put_itimerval(), which copies a stack-allocated __kernel_old_itimerval structure to userspace without zero-initializing the aggregate first. On architectures such as native sparc64, struct __kernel_old_timeval contains a four-byte padding hole after tv_usec because tv_sec is 64-bit while __kernel_suseconds_t is 32-bit. When getitimer() returns to user mode, uninitialized stack bytes in these padding holes leak across the kernel/userspace boundary.
Critical Impact
Local unprivileged processes can read residual kernel stack contents through getitimer(), potentially exposing sensitive kernel memory used in address-space layout defenses.
Affected Products
- Linux kernel branches receiving the timers/itimer fix (see referenced stable commits)
- Architectures with padding in __kernel_old_timeval, notably native sparc64
- Any distribution shipping affected kernel versions prior to the backported fixes
Discovery Timeline
- 2026-09-11 - CVE-2026-89765 published to the National Vulnerability Database
- 2026-09-14 - Last updated in NVD database
Technical Details for CVE-2026-89765
Vulnerability Analysis
The vulnerability is an uninitialized memory disclosure in the kernel's interval timer path. put_itimerval() allocates a __kernel_old_itimerval on the kernel stack, populates only the named fields, and copies the entire structure to userspace via copy_to_user(). On sparc64, the compiler inserts a four-byte hole after tv_usec within each embedded __kernel_old_timeval. Because the outer aggregate is never zeroed, the padding bytes retain whatever data previously occupied that stack slot.
When a userspace process invokes getitimer(), those padding bytes travel back with the returned timer values. An attacker can call getitimer() repeatedly and harvest kernel stack content, potentially recovering pointers, canaries, or fragments of sensitive data used by earlier system calls on the same task.
Root Cause
The root cause is a mismatch between the layout of __kernel_old_timeval and the initialization pattern used by put_itimerval(). Field-wise assignment does not touch implicit padding. On 64-bit architectures where tv_sec is 64-bit and __kernel_suseconds_t is 32-bit, the padding is deterministic in size but non-deterministic in content, satisfying the conditions for [CWE-908 Use of Uninitialized Resource] and [CWE-200 Exposure of Sensitive Information].
Attack Vector
Exploitation requires only local, unprivileged code execution on an affected kernel. The attacker sets up an interval timer with setitimer() and then repeatedly invokes getitimer(), reading the returned buffer and extracting bytes at the padding offsets. No special capability, container escape, or race condition is required.
The fix, applied across the referenced stable commits, zero-initializes the aggregate before assigning the named fields so that padding is deterministic before the copy crosses the user/kernel boundary. See the kernel stable commit 00ef529 for the reference change.
Detection Methods for CVE-2026-89765
Indicators of Compromise
- No network-visible indicators exist because exploitation occurs entirely within a local process via legitimate system calls.
- Unusual volumes of getitimer() invocations from unprivileged processes, especially when paired with minimal timer usage, may indicate stack-scraping behavior.
Detection Strategies
- Audit running kernel versions against the fixed commits listed in the NVD references to identify unpatched hosts.
- Use kernel address sanitizer (KASAN) or KMSAN builds in test environments to surface uninitialized memory reads in timer paths.
- Deploy syscall auditing (auditd, eBPF) to record processes making high-frequency getitimer() calls without corresponding timer activity.
Monitoring Recommendations
- Track kernel package versions across the fleet and alert on hosts running Linux kernels older than the patched stable releases.
- Monitor sparc64 and other 64-bit systems with padded timeval layouts more aggressively, as they are the primary leak surface.
- Correlate abnormal getitimer() telemetry with process lineage to identify local reconnaissance activity.
How to Mitigate CVE-2026-89765
Immediate Actions Required
- Apply the vendor-supplied kernel updates that incorporate the timers/itimer zero-initialization fix.
- Prioritize patching on sparc64 systems and any 64-bit hosts running workloads that grant local shell access to untrusted users.
- Rebuild and redeploy any custom kernels pulling from affected stable branches after cherry-picking the referenced commits.
Patch Information
The fix zero-initializes the __kernel_old_itimerval aggregate before assigning fields, ensuring implicit padding is deterministic prior to copy_to_user(). Patches have been merged across multiple stable branches. Reference commits include 00ef529, 18c7d85, 2bf5e8f, 4c66719, 4ea7014, 6d4b937, a17b345, and bdfda99.
Workarounds
- No supported runtime workaround exists because getitimer() is a standard POSIX interface required by many applications.
- Restrict local shell access on affected hosts until patched kernels are deployed, reducing exposure to unprivileged attackers.
- Where feasible, migrate workloads off sparc64 to architectures where the padding hole does not manifest.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.