CVE-2026-98301 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mst: move switchdev call outside rcu
This is a follow-up of one of sashiko's pre-existing bug reports. br_mst_set_state() calls switchdev_port_attr_set() for nonzero MSTIs while holding rcu_read_lock() which invokes the blocking switchdev notifier chain and may sleep. Nonzero MSTI changes come from netlink with rtnl held. Move the switchdev call before entering the rcu section and assert that rtnl is held.
The call cannot be deferred because netlink needs its error and extack. Also DSA reads the old bridge MST state during the callback and checks it. A deferred callback will be late and will see the updated state.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel vulnerability identified in the networking subsystem involves a concurrency violation within the Multiple Spanning Tree (MST) implementation of the Ethernet bridge code. Specifically, the function br_mst_set_state() was invoking switchdev_port_attr_set() while holding an RCU read-side lock. This architectural flaw creates a critical operational risk because the underlying switchdev notifier chain is designed to be blocking and may trigger operations that require sleeping or acquiring mutexes. In kernel programming contexts, executing potentially blocking code within an RCU read section violates fundamental concurrency rules, as RCU readers are expected to remain non-blocking to ensure scalability and prevent deadlocks when other parts of the system attempt to synchronize with grace periods or update data structures protected by the same lock.
This issue stems from a pre-existing bug report where the sequence of operations did not properly separate locking domains. The vulnerability arises because netlink requests that modify MST instances are processed while holding the rtnl_lock, which provides necessary serialization for configuration changes. However, the code incorrectly entered an RCU critical section before performing the switchdev attribute set operation. Since nonzero MSTI changes originate from these netlink interactions, moving the switchdev call outside of the RCU read lock ensures that blocking operations do not interfere with RCU grace periods. The fix involves reordering the logic so that the switchdev notification occurs prior to entering the RCU section, while simultaneously asserting that the rtnl_lock is held to maintain proper synchronization for configuration updates.
The decision to move this call rather than defer it is driven by specific functional requirements inherent to Distributed Switch Architecture (DSA) and netlink error handling mechanisms. Netlink interfaces require immediate feedback regarding success or failure of attribute changes, including extended acknowledgment data that must be returned to the user space application immediately upon processing. Furthermore, DSA drivers rely on reading the previous bridge MST state during their callback execution to validate transitions correctly. If the switchdev call were deferred using RCU callbacks, it would execute asynchronously after the original context has changed, causing the driver to observe an updated state rather than the intended old state, leading to incorrect logic or validation failures in hardware offloading scenarios.
From a security and stability perspective, this vulnerability falls under CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization. The improper use of RCU locks around blocking operations can lead to kernel panics, deadlocks, or unpredictable system behavior when multiple threads attempt to access shared network bridge structures simultaneously. In the context of the MITRE ATT&CK framework, this type of flaw is relevant to techniques involving exploitation of software vulnerabilities for denial of service, as a triggered deadlock could render network interfaces unresponsive and disrupt critical infrastructure services dependent on stable kernel networking stacks.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the locking order in net/bridge/br_mst.c. System administrators should ensure their systems are updated with kernels containing this fix to prevent potential denial-of-service conditions arising from race conditions or deadlocks during MST configuration changes. For developers and maintainers, adherence to strict RCU usage guidelines is essential; specifically, avoiding any potentially sleeping calls within read-side critical sections unless explicitly permitted by the API documentation. Regular code audits focusing on lock ordering and synchronization primitives in network subsystems can help identify similar vulnerabilities before they are deployed into production environments.