CVE-2026-53188 Overview
CVE-2026-53188 is a Linux kernel vulnerability in the RDMA (Remote Direct Memory Access) core subsystem. The flaw resides in the ib_get_ucaps() function, which previously relied solely on the device number (dev_t) to validate user capability character device file descriptors. Because character and block devices can share the same dev_t value through aliasing, an attacker controlling a block device with a matching dev_t could masquerade as an authentic ucap cdev file descriptor. The fix validates the file operations (f_ops) structure to ensure only legitimate character devices are accepted.
Critical Impact
A local user can substitute a block device for a ucap character device, bypassing capability validation in the RDMA subsystem and potentially gaining unauthorized RDMA capabilities.
Affected Products
- Linux kernel versions containing the RDMA/core ib_get_ucaps() implementation prior to the referenced stable commits
- Distributions shipping affected upstream kernel revisions
- RDMA/InfiniBand-enabled Linux systems exposing user capability cdev interfaces
Discovery Timeline
- 2026-06-25 - CVE-2026-53188 published to NVD
- 2026-06-25 - Last updated in NVD database
Technical Details for CVE-2026-53188
Vulnerability Analysis
The vulnerability is a type confusion / improper input validation flaw in the Linux kernel RDMA core. The ib_get_ucaps() function accepts a file descriptor from userspace and attempts to confirm it references a ucap character device. The original implementation compared only the dev_t (device number) of the underlying inode against expected values.
Linux maintains separate namespaces for character devices and block devices, but both use dev_t identifiers. The same numeric dev_t can therefore exist as both a character device and a block device simultaneously. A local attacker able to create or open a block device with a dev_t matching a ucap character device passes the original check despite providing the wrong device class.
Root Cause
The root cause is reliance on a non-unique identifier (dev_t) to authenticate kernel object types. The validation logic did not test the file's f_ops pointer, which is the authoritative indicator of the backing driver and device type. Without f_ops comparison, character/block aliasing defeats the check.
Attack Vector
An unprivileged local user opens a block device whose dev_t collides with a ucap character device and passes the resulting file descriptor to an RDMA syscall path that invokes ib_get_ucaps(). The kernel accepts the descriptor as a valid ucap reference, allowing the attacker to influence subsequent capability handling within the RDMA subsystem. The fix introduces an explicit f_ops equality check so only authentic ucap cdevs are accepted. Technical detail is available in the kernel commit aa18128, commit 96b6e98, and commit 4a1b1ac.
Detection Methods for CVE-2026-53188
Indicators of Compromise
- Unexpected processes opening RDMA/InfiniBand device nodes under /dev/infiniband/ followed by syscalls referencing block devices
- Userspace binaries invoking ucap-related ioctls from unprivileged contexts that previously had no RDMA workload
- Audit log entries showing block device file descriptors passed to RDMA-related kernel interfaces
Detection Strategies
- Enable kernel auditing on RDMA character devices and correlate open() calls against expected RDMA workloads
- Use auditd rules to log file descriptor passing into RDMA syscalls and flag descriptors whose inode is backed by a block device
- Monitor kernel version inventory and compare against the fixed stable commits to identify unpatched hosts
Monitoring Recommendations
- Track loaded kernel modules for ib_uverbs, ib_core, and related RDMA components on production hosts
- Alert on attempts by non-root users to open RDMA character devices outside known service accounts
- Centralize kernel logs and audit trails to detect anomalous device access patterns across the fleet
How to Mitigate CVE-2026-53188
Immediate Actions Required
- Inventory all Linux hosts running RDMA/InfiniBand workloads and identify kernels missing the upstream fix
- Apply distribution kernel updates that incorporate the referenced stable commits
- Restrict access to /dev/infiniband/ device nodes to only the users and services that require RDMA
Patch Information
The vulnerability is resolved in the upstream Linux kernel by validating f_ops in ib_get_ucaps(). Apply the fix from kernel commit 4a1b1ac, kernel commit 96b6e98, or kernel commit aa18128, or upgrade to a distribution kernel that has merged these stable backports.
Workarounds
- Tighten permissions on RDMA character device nodes so only trusted service accounts can open them
- Unload RDMA kernel modules (ib_uverbs, ib_core) on systems that do not require RDMA functionality
- Use mandatory access control policies (SELinux, AppArmor) to restrict which processes may invoke RDMA syscalls
# Example: restrict access to RDMA device nodes and unload unused modules
sudo chmod 0600 /dev/infiniband/*
sudo chown root:rdma /dev/infiniband/*
# If RDMA is not required on the host
sudo modprobe -r ib_uverbs
sudo modprobe -r ib_core
# Verify running kernel includes the fix
uname -r
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
