CVE-2026-90108 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
net/smc: free stashed qentry before overwrite in REQ_ADD_LINK to ADD_LINK transition
When smc_llc_event_handler() transitions the local LLC flow from SMC_LLC_FLOW_REQ_ADD_LINK to SMC_LLC_FLOW_ADD_LINK on arrival of an ADD_LINK request, it calls smc_llc_flow_qentry_set() unconditionally:
if (lgr->llc_flow_lcl.type == SMC_LLC_FLOW_REQ_ADD_LINK) {
lgr->llc_flow_lcl.type = SMC_LLC_FLOW_ADD_LINK; smc_llc_flow_qentry_set(&lgr->llc_flow_lcl, qentry); ... }
A CONFIRM_LINK or ADD_LINK_CONT arriving while flow->type is SMC_LLC_FLOW_REQ_ADD_LINK is stashed into flow->qentry via the SMC_LLC_CONFIRM_LINK / SMC_LLC_ADD_LINK_CONT handler (which stores into flow->qentry for any non-NONE flow type). When the subsequent ADD_LINK arrives, the REQ_ADD_LINK branch overwrites flow->qentry with the new pointer without first freeing the stashed allocation, leaking one kmalloc object.
The stashed entry has no consumer: smc_llc_wait() is only called from llc_add_link_work, which is not yet scheduled while the flow type remains REQ_ADD_LINK. No waiter is sleeping on llc_msg_waiter at this point. It is safe to unconditionally free any stashed qentry before the overwrite.
Call smc_llc_flow_qentry_del() before smc_llc_flow_qentry_set() in the REQ_ADD_LINK branch. smc_llc_flow_qentry_del() already checks flow->qentry before freeing, so the normal path where no entry is stashed is a no-op.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's Shared Memory Communications over RDMA (SMC-R) implementation contains a memory leak vulnerability within its Link Control Protocol state machine logic. This flaw specifically affects the transition process when establishing new links between SMC peers, occurring in the net/smc subsystem where link group management and LLC flow control are handled. The issue arises during the handling of ADD_LINK requests that trigger a change in the local LLC flow type from REQ_ADD_LINK to ADD_LINK. In this specific state transition scenario, the kernel fails to properly manage previously allocated memory structures associated with pending messages, leading to an unbounded accumulation of leaked kernel memory objects over time if such link establishment attempts occur repeatedly.
The technical root cause lies in the unconditional overwrite of a pointer within the flow structure without prior deallocation of any existing data it references. When an ADD_LINK request arrives and the local LLC flow is currently in the REQ_ADD_LINK state, the smc_llc_event_handler function executes code that updates the flow type to ADD_LINK and immediately calls smc_llc_flow_qentry_set with a new qentry pointer. However, prior to this transition, if CONFIRM_LINK or ADD_LINK_CONT messages arrived while the flow was still in the REQ_ADD_LINK state, they were stashed into the same flow->qentry field via their respective handlers. These handlers store the incoming message queue entry for later processing because the link establishment is not yet complete. Consequently, when the final ADD_LINK request arrives and triggers the transition to ADD_LINK, the code overwrites the pointer in flow->qentry with a new value without first freeing the previously stashed allocation. This results in a direct loss of reference to that kmalloc-allocated object, rendering it unreachable by any part of the kernel memory management subsystem.
The operational impact of this vulnerability is primarily characterized as a denial-of-service condition through resource exhaustion rather than immediate privilege escalation or data leakage. Because the leaked qentry has no active consumer at the time of the leak, there are no concurrent waiters sleeping on llc_msg_waiter that would expect to process these stashed messages. The function smc_llc_wait is only invoked from llc_add_link_work, which itself is not scheduled until after the flow type transitions away from REQ_ADD_LINK and the link setup proceeds further. Therefore, while the leaked memory does not cause immediate functional correctness issues or crashes in a single instance, it represents a steady drain on kernel heap resources. In environments with high churn of SMC link establishments, this can lead to progressive degradation of system performance due to increased pressure on the slab allocator and potential eventual out-of-memory conditions if left unmitigated over extended periods.
From a classification perspective, this vulnerability aligns with CWE-401, which describes missing release of memory after effective usage, commonly referred to as a memory leak. It also relates to improper resource management during state transitions within network protocol implementations. In the context of the MITRE ATT&CK framework, while not directly exploitable for remote code execution or unauthorized access in its current form, such vulnerabilities can be leveraged in conjunction with other flaws to facilitate denial-of-service attacks against critical infrastructure services relying on high-performance RDMA-based communication stacks. The lack of proper cleanup during state machine transitions is a common pattern that security auditors often flag as requiring remediation to maintain system stability and reliability.
The recommended mitigation involves modifying the smc_llc_event_handler function in net/smc/llc.c to ensure proper resource lifecycle management. Specifically, developers must insert a call to smc_llc_flow_qentry_del before invoking smc_llc_flow_qentry_set within the REQ_ADD_LINK branch of the state transition logic. The existing implementation of smc_llc_flow_qentry_del is designed safely; it checks whether flow->qentry points to a valid allocation before attempting to free it, ensuring that normal paths where no entry was stashed remain unaffected and operate as a no-op. This patch ensures that any previously stashed qentries are correctly released back to the kernel memory pool before being overwritten by new pointers during the link addition process. Applying this fix restores proper resource accounting and prevents the gradual accumulation of leaked kmalloc objects, thereby maintaining the long-term stability and security posture of systems utilizing SMC-R functionality.