CVE-2026-53294 Overview
CVE-2026-53294 is a double-free vulnerability in the Linux kernel mailbox-test driver. The flaw occurs when the receive (RX) channel is aliased to the transmit (TX) channel while using a different memory-mapped I/O (MMIO) region. During channel teardown, the shared channel reference is freed twice, corrupting kernel memory allocator state. The upstream fix instructs the driver not to free the reused channel a second time.
Critical Impact
A double-free in kernel memory management can lead to memory corruption, denial of service, or potential local privilege escalation on affected Linux systems.
Affected Products
- Linux kernel builds that include the mailbox-test driver
- Distributions shipping vulnerable stable branches prior to the referenced fix commits
- Embedded and server platforms exercising aliased RX/TX mailbox channels with separate MMIO regions
Discovery Timeline
- 2026-06-26 - CVE-2026-53294 published to NVD
- 2026-06-30 - Last updated in NVD database
Technical Details for CVE-2026-53294
Vulnerability Analysis
The mailbox-test driver in the Linux kernel supports test configurations where a single mailbox channel serves both TX and RX operations. When the RX channel is defined as an alias of the TX channel but references a different MMIO region, the driver stores two pointers that ultimately reference the same underlying channel object.
During driver unbind or error unwind, the cleanup path releases the RX channel and then releases the TX channel independently. Because both point to the same object, the second release operates on already-freed memory, producing a double-free condition in the kernel slab allocator.
The issue is a memory safety defect in kernel driver code and falls under the Double Free class of vulnerabilities. It requires the affected code path to be reached, which typically involves loading the driver with a matching device tree or platform configuration.
Root Cause
The root cause is missing logic to detect that the RX channel is an alias of the TX channel prior to invoking mbox_free_channel(). The driver treats the two pointers as distinct resources, so both are freed on teardown. The upstream patches add a conditional check that skips the second free when the pointers reference the same channel.
Attack Vector
Exploitation requires local access on a system where the mailbox-test driver is loaded and configured with an aliased RX/TX channel. An attacker capable of triggering driver unbind or a failure path during probe can reach the double-free. The realistic outcome is kernel memory corruption leading to denial of service; escalation to arbitrary code execution would require chaining with a separate primitive to control freed allocations.
No public proof-of-concept exploit and no evidence of exploitation in the wild are recorded for this identifier.
Detection Methods for CVE-2026-53294
Indicators of Compromise
- Kernel oops or panic messages referencing mailbox-test, mbox_free_channel, or slab double-free warnings such as kernel BUG at mm/slub.c
- Repeated KASAN: double-free or invalid-free reports involving mailbox channel structures
- Unexpected reboots or hangs during driver unbind of mailbox-test
Detection Strategies
- Enable Kernel Address Sanitizer (KASAN) on test builds to surface double-free conditions during driver load and unload cycles
- Compare running kernel versions against the fixed commits referenced in the upstream stable tree
- Audit device tree and platform data for mailbox-test nodes that alias RX to TX with distinct MMIO ranges
Monitoring Recommendations
- Forward kernel ring buffer (dmesg) and journalctl -k output to a centralized log platform and alert on slab corruption and KASAN signatures
- Track kernel package versions across fleets and flag hosts still running pre-patch stable branches
- Monitor uptime anomalies and crash dumps on systems that load the mailbox-test driver
How to Mitigate CVE-2026-53294
Immediate Actions Required
- Apply the vendor kernel update that includes one of the upstream fix commits: 240c71a, 3afca89f, 5c209299, 5d4f3d0f, 82f6dcea, 88ebadbf, c494a11d, or fc0089f8
- Inventory systems that build or load the mailbox-test driver and prioritize them for patching
- Restrict local access on affected hosts until the kernel is updated
Patch Information
The upstream Linux stable tree includes fixes that modify the mailbox-test driver cleanup logic to skip freeing the RX channel when it is aliased to the TX channel. Downstream distributions should incorporate the corresponding backports into their supported kernel branches. Rebuild any custom kernels from sources that include the referenced commits.
Workarounds
- Unload the mailbox-test module and blacklist it if it is not required for platform functionality
- Avoid configurations that alias the RX channel to the TX channel while assigning different MMIO regions
- Limit driver access by restricting CAP_SYS_MODULE and controlling who can trigger driver bind or unbind events through sysfs
# Blacklist the mailbox-test module until the kernel is patched
echo "blacklist mailbox-test" | sudo tee /etc/modprobe.d/blacklist-mailbox-test.conf
sudo rmmod mailbox_test 2>/dev/null || true
sudo update-initramfs -u
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
