CVE-2026-31493: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31493 Overview

CVE-2026-31493 is a use-after-free vulnerability [CWE-416] in the Linux kernel's RDMA/efa (Elastic Fabric Adapter) driver. The flaw occurs in the admin queue completion handling path. When an admin command completes with an error, the driver prints data from the completion context after that context has already been freed by the polling or interrupt handler. The freed memory may have been reallocated and reused, leading to reads from memory in an unknown state.

The vulnerability affects multiple stable branches of the Linux kernel, including releases beginning at 5.12 and pre-release 7.0 candidates. The fix re-orders allocation and deallocation so they are symmetric, ensuring the context remains valid for any potential use.

Critical Impact

Local low-privileged users on systems with the EFA RDMA driver loaded can trigger reads from freed kernel memory, potentially leading to memory corruption, information disclosure, or local privilege escalation.

Affected Products

• Linux Kernel 5.12 (stable branch)
• Linux Kernel 7.0 release candidates (rc1 through rc7)
• Systems using the RDMA/efa driver for Elastic Fabric Adapter

Discovery Timeline

• 2026-04-22 - CVE-2026-31493 published to NVD
• 2026-04-28 - Last updated in NVD database

Technical Details for CVE-2026-31493

Vulnerability Analysis

The vulnerability resides in the admin queue submission and completion flow of the efa RDMA driver. The driver processes administrative commands asynchronously using a completion context structure that tracks each in-flight command.

When the polling or interrupt completion handler processes a finished command, it deallocates the completion context. However, the calling code path subsequently dereferences that same context to print diagnostic data if the command returned an error. By that point, the memory has been freed and may have been reissued for an unrelated allocation.

Reading from this freed allocation produces undefined behavior. An attacker who can influence kernel allocations could spray controlled data into the freed slot, turning the diagnostic read into an information disclosure primitive or, depending on subsequent operations on the stale pointer, a memory corruption primitive.

Root Cause

The defect is an asymmetric allocation/deallocation pattern in the admin submission flow. The completion context is freed inside the completion handler before all consumers have finished accessing it. The patch restructures the flow so that allocation and deallocation are paired symmetrically and the free occurs only after all reads of the context have completed.

Attack Vector

Exploitation requires local access with low privileges on a host that loads the efa driver — typically AWS EC2 instances supporting Elastic Fabric Adapter or similar RDMA-enabled hardware. An attacker triggers admin commands that complete with errors while racing the completion handler, causing the kernel to read from a freed and potentially attacker-controlled allocation. No user interaction is required.

No verified public exploit code is available for this issue. Refer to the upstream commits in the Kernel Git Commit Log for the precise patch hunks.

Detection Methods for CVE-2026-31493

Indicators of Compromise

• Unexpected kernel oops or warnings referencing the efa driver in dmesg or /var/log/kern.log
• KASAN reports identifying use-after-free reads inside efa_com admin completion functions
• Crashes or instability on RDMA-enabled instances correlated with admin command error paths

Detection Strategies

• Enable KASAN on test kernels to surface use-after-free reads in the EFA admin path during fuzzing or stress testing
• Inventory hosts with the efa module loaded using lsmod | grep efa and compare kernel versions against patched commits 0dd98aea1c0c, 1cf95fe5dc54, and ef3b06742c8a
• Monitor kernel ring buffer telemetry for repeated EFA admin command failures, which may precede exploitation attempts

Monitoring Recommendations

• Forward dmesg and kernel audit logs to a central SIEM for correlation across the fleet
• Alert on local privilege escalation indicators such as new SUID processes or unexpected uid=0 shells originating from non-administrative users
• Track loaded kernel modules and kernel version drift on RDMA-capable cloud instances

How to Mitigate CVE-2026-31493

Immediate Actions Required

• Identify systems running affected Linux kernel versions with the efa driver loaded, prioritizing AWS EC2 instances using Elastic Fabric Adapter
• Apply the upstream stable kernel updates containing commits 0dd98aea1c0c45987fa2dd92f988b0eb1a72c125, 1cf95fe5dc5471efea947b4c6f8913da6bc7976e, and ef3b06742c8a201d0e83edc9a33a89a4fe3009f8
• Restrict local shell access on affected hosts to trusted administrators until patches are deployed

Patch Information

The fix is available as three upstream commits in the Linux kernel stable tree. The patch changes the admin submission flow so that the completion context allocation and deallocation are symmetric, with deallocation deferred until after all uses of the context. See the Kernel Git Commit Log for the canonical fix and the additional stable backport commits listed under vendor advisories.

Workarounds

• Unload the efa kernel module on systems that do not require Elastic Fabric Adapter functionality using modprobe -r efa
• Blacklist the efa module via /etc/modprobe.d/ to prevent automatic loading on reboot where the driver is unnecessary
• Limit local user accounts and enforce least privilege to reduce the attack surface for local exploitation

# Prevent the efa driver from loading where not required
echo "blacklist efa" | sudo tee /etc/modprobe.d/blacklist-efa.conf
sudo modprobe -r efa

# Verify current kernel version against patched releases
uname -r