CVE-2026-98284 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
netlink: do not free nlk->groups while lockless readers can use it
netlink_realloc_groups() uses krealloc() under netlink_table_grab(). Whenever NLGRPSZ(groups) lands in a different kmalloc bucket, the old bitmap is freed immediately.
Two readers of nlk->groups / nlk->ngroups do not hold the netlink table lock:
1) sk_diag_dump_groups(). Hashed (bound) sockets are dumped from the rhashtable walk in __netlink_diag_dump(), which only holds RCU. Only the mc_list part of the dump takes nl_table_lock.
2) netlink_native_seq_show() (/proc/net/netlink), whose walk has been lockless since commit 21e4902aea80 ("netlink: Lockless lookup with RCU grace period in socket release").
Both can read a freed buffer, and sk_diag_dump_groups() can also read past the end of the old (smaller) buffer if it happens to load the old @groups pointer together with the new @ngroups value, copying the result into a NETLINK_DIAG_GROUPS attribute.
This is the same class of bug that commit f773608026ee ("netlink: access nlk groups safely in netlink bind and getname") fixed for bind() and getname(); these two readers were missed. Simply grabbing the table lock in sk_diag_dump_groups() is not an option, because it is also called with nl_table_lock already held from the mc_list section of the dump.
Make the lockless readers safe instead:
- Allocate a new bitmap and free the old one after an RCU grace period, instead of relying on the implicit kfree() done by krealloc().
- Publish @groups before @ngroups, both with release semantics, and have the lockless readers load @ngroups first. A reader can then never pair the new (bigger) size with the old (smaller) buffer, and a reader picking up the new pointer while still seeing the old size is guaranteed to see the initialized bitmap.
netlink_realloc_groups() is called from process context (bind() and setsockopt()), so kfree_rcu_mightsleep() can be used, once the table has been released.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel netlink subsystem contains a race condition vulnerability related to concurrent access of socket group bitmaps during reallocation operations. This issue arises within the netlink_realloc_groups function, which is responsible for resizing the bitmap that tracks multicast groups associated with a netlink socket. The implementation utilizes krealloc to manage memory allocation changes. Under specific conditions where the new size falls into a different kmalloc bucket than the old one, krealloc frees the original buffer immediately while returning a pointer to newly allocated memory. This immediate deallocation creates a window of vulnerability because it does not account for lockless readers that may still hold references to the old bitmap data structure.
Two specific code paths access nlk->groups and nlk->ngroups without holding the netlink table lock, relying instead on RCU read-side critical sections for synchronization. The first path is sk_diag_dump_groups, which is invoked during diagnostic dumps of hashed sockets via __netlink_diag_dump. This function performs a walk through an rhashtable protected only by RCU mechanisms. While certain parts of this dump process acquire the nl_table_lock, specifically when handling mc_list data, other sections operate locklessly. The second path involves netlink_native_seq_show, which exposes netlink socket information via /proc/net/netlink. This interface has utilized a lockless lookup mechanism with an RCU grace period since commit 21e4902aea80 to improve performance and reduce contention.
The core technical flaw lies in the potential for use-after-free scenarios due to unsynchronized updates between writers and these specific readers. When netlink_realloc_groups executes, it may free the old bitmap buffer before RCU grace periods have elapsed for ongoing lockless reads. Consequently, sk_diag_dump_groups can read from a memory region that has already been returned to the kernel allocator or freed entirely. Furthermore, there is an additional risk of out-of-bounds access if a reader loads the new pointer value for nlk->groups while simultaneously reading the old, smaller value for nlk->ngroups. In such a case, the reader might attempt to copy data based on the larger group count but using the address of the now-freed or reallocated buffer, leading to memory corruption or information disclosure via NETLINK_DIAG_GROUPS attributes.
This vulnerability class mirrors issues previously addressed in commit f773608026ee, which corrected unsafe access patterns during netlink bind and getname operations by ensuring proper synchronization. However, the lockless readers identified here were inadvertently omitted from that remediation effort. Standard mitigation strategies such as acquiring the nl_table_lock within sk_diag_dump_groups are not viable because this function is often called in contexts where the lock is already held for other parts of the dump operation, which would result in a deadlock situation given the locking hierarchy constraints.
The resolution involves restructuring how memory reallocation and publication occur to ensure safe concurrent access without requiring additional locks that could cause deadlocks or performance degradation. The fix mandates allocating a new bitmap while retaining the old one until an RCU grace period has passed, thereby preventing immediate deallocation of buffers still in use by readers. This is achieved using kfree_rcu_mightsleep, which safely defers freeing memory to process context after all pre-existing read-side critical sections have completed. Additionally, the update sequence for nlk->groups and nlk->ngroups is modified to enforce a specific publication order with release semantics. By publishing the new groups pointer before updating ngroups, readers that load ngroups first are guaranteed to either see both old values or both new values, eliminating the possibility of pairing a larger size count with an older buffer address.
From a standards perspective, this vulnerability aligns with CWE-416 Use After Free and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization Race Condition. The attack vector leverages timing differences in lockless RCU read paths to access freed memory, which can be exploited for denial of service through kernel crashes or potentially for privilege escalation if the corrupted data leads to arbitrary code execution pathways. In terms of MITRE ATT&CK mapping, this falls under T1059 Command and Scripting Interpreter via system-level APIs, specifically targeting network configuration interfaces that are often accessible by local users with varying privileges. The exploitation relies on precise timing or repeated triggering of group reallocation events while diagnostic dumps or procfs reads are active concurrently.
Mitigation strategies for administrators include applying the kernel patch that implements RCU-safe memory management and ordered publication semantics as described in the fix. For systems where immediate patching is not feasible, reducing the frequency of netlink socket group modifications can minimize the window of exposure. Monitoring system logs for unexpected oops or panic messages related to netlink operations may help detect exploitation attempts. It is also advisable to restrict access to /proc/net/netlink and diagnostic tools like ss or ip command groups if they are not required by non-privileged users, thereby limiting the attack surface available to potential adversaries attempting to trigger this race condition.