CVE-2026-63837: Linux Kernel Information Disclosure Bug

CVE-2026-63837 Overview

CVE-2026-63837 is an information disclosure vulnerability in the Linux kernel's Elastic Network Adapter (ENA) driver. The flaw exists in the PTP Hardware Clock (PHC) subsystem, specifically in the ena_phc_gettimex64() function. The function writes the timestamp output parameter regardless of whether the underlying ena_com_phc_get_timestamp() call succeeded or failed. When the hardware call fails — for example, when PHC is disabled or in a blocked state — the timestamp buffer may contain uninitialized kernel stack memory or invalid hardware values. That data is then passed to userspace through the PTP ioctl interface.

Critical Impact
Local users can retrieve uninitialized kernel stack memory through the PTP ioctl interface on affected AWS ENA-equipped Linux systems, enabling potential information leakage of sensitive kernel data.

Affected Products

Linux kernel versions containing the ENA driver with PHC support prior to the fix commits
Systems running the AWS Elastic Network Adapter (ENA) driver with PTP hardware clock enabled
Distributions shipping affected mainline and stable kernel branches

Discovery Timeline

2026-07-19 - CVE-2026-63837 published to NVD
2026-07-19 - Last updated in NVD database

Technical Details for CVE-2026-63837

Vulnerability Analysis

The defect resides in the ENA driver's PTP Hardware Clock implementation. The ena_phc_gettimex64() handler is invoked when userspace calls the PTP clock ioctl to read the current hardware timestamp. Internally, the handler acquires a lock, calls ena_com_phc_get_timestamp() to retrieve the timestamp from device hardware, releases the lock, and then copies the result into the caller-supplied timespec64 output.

The correctness gap is that the handler does not check the return code from ena_com_phc_get_timestamp() before assigning the output. When the hardware call fails — because PHC is disabled, blocked, or the device returns an error — the timestamp buffer is left in its prior state. That state is either uninitialized stack memory or stale hardware values. The kernel then copies these bytes to userspace through the PTP ioctl path, producing both a security issue and a functional bug that returns invalid time values.

This pattern is classified as [CWE-908] Use of Uninitialized Resource and [CWE-200] Exposure of Sensitive Information. Kernel stack contents commonly include pointers, return addresses, and residue from prior syscalls, which are useful to attackers attempting to defeat Kernel Address Space Layout Randomization (KASLR).

Root Cause

The root cause is missing error handling. The function unconditionally writes the output parameter instead of gating the write on a successful return from ena_com_phc_get_timestamp(). The upstream fix reorders the logic so the return code is checked after the lock is released, and the output timestamp is populated only on success.

Attack Vector

A local unprivileged user with access to /dev/ptp* character devices on a host using the ENA driver can trigger the vulnerability by issuing PTP_SYS_OFFSET_PRECISE or equivalent ioctl requests while PHC is disabled or in a blocked state. Each failed hardware read returns kernel memory bytes to the caller. Repeated invocations allow an attacker to sample kernel stack content over time.

The vulnerability is not remotely exploitable and does not permit code execution. Its impact is confined to confidentiality of kernel memory.

Detection Methods for CVE-2026-63837

Indicators of Compromise

Unusual frequency of PTP ioctl calls (PTP_SYS_OFFSET, PTP_SYS_OFFSET_PRECISE, PTP_SYS_OFFSET_EXTENDED) issued by non-privileged processes
Unexpected user processes opening /dev/ptp0 or other PTP character devices on ENA-equipped AWS instances
Kernel logs showing ENA PHC errors or blocked-state transitions coinciding with repeated userspace timestamp reads

Detection Strategies

Audit installed kernel package versions against the fix commits 24a08d7, bddf598, and edcb049 from the stable tree
Monitor auditd for openat and ioctl syscalls targeting PTP device nodes originating from unexpected UIDs
Correlate ENA driver diagnostic messages indicating PHC disabled or blocked states with concurrent PTP userspace activity

Monitoring Recommendations

Enable Linux audit rules covering /dev/ptp* access and PTP-related ioctl numbers on AWS EC2 instances
Collect kernel ring buffer messages from the ENA driver and forward them to a central log platform for correlation
Baseline expected PTP clients (chronyd, ptp4l) and alert on any process outside that baseline reading PHC timestamps

How to Mitigate CVE-2026-63837

Immediate Actions Required

Apply the stable kernel updates that include commits 24a08d7d6218, bddf59818ae5, and edcb049d836e from the upstream Linux tree
Rebuild or update distribution kernels on all AWS EC2 instances that use the ENA driver with PTP hardware clock support
Restrict access to /dev/ptp* device nodes to trusted service accounts using file permissions and Linux capabilities

Patch Information

The fix is available in the Linux stable tree. Refer to the Kernel Git Commit 24a08d7, Kernel Git Commit bddf598, and Kernel Git Commit edcb049 for the applicable stable branches. The patch checks the return code from ena_com_phc_get_timestamp() after releasing the lock and only writes the timestamp on success.

Workarounds

Disable the ENA PHC feature on affected instances if PTP hardware timestamping is not required for workloads
Tighten permissions on /dev/ptp* so only privileged time-synchronization services can open the device
Remove unnecessary PTP client packages from hosts that do not need hardware time synchronization

bash

# Restrict PTP device access to a dedicated group
sudo groupadd ptpusers
sudo chown root:ptpusers /dev/ptp0
sudo chmod 640 /dev/ptp0

# Verify installed kernel includes the fix commits
uname -r
dpkg -l | grep linux-image   # Debian/Ubuntu
rpm -qa | grep kernel         # RHEL/Amazon Linux