CVE-2026-93226 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv6: use RCU iterator to dump route exceptions
rt6_nh_dump_exceptions() uses hlist_for_each_entry() to iterate over RCU-protected exception lists. The caller holds rcu_read_lock(), but does not hold rt6_exception_lock, so rt6_insert_exception() can concurrently add an entry with hlist_add_head_rcu().
KCSAN reports this race (irrelevant details omitted):
================================================================== BUG: KCSAN: data-race in rt6_insert_exception / rt6_nh_dump_exceptions
write (marked) to 0xffff8a7c44c59620 of 8 bytes by interrupt on cpu 5: rt6_insert_exception+0x3bb/0x760 __ip6_rt_update_pmtu+0x4fe/0x750 ip6_sk_update_pmtu+0x19a/0x3b0 udpv6_err+0x3ff/0x800 icmpv6_notify+0x1e1/0x440 icmpv6_rcv+0x8c0/0xab0 ip6_protocol_deliver_rcu+0x616/0x840 ip6_input_finish+0xb9/0x160 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f
read to 0xffff8a7c44c59620 of 8 bytes by task 549 on cpu 14: rt6_nh_dump_exceptions+0xb3/0x260 rt6_dump_route+0x53e/0x5f0 fib6_dump_node+0x6d/0xf0 fib6_walk_continue+0x290/0x2d0 fib6_dump_table+0x28d/0x360 inet6_dump_fib+0x37d/0x620 rtnl_dumpit+0x7b/0xd0 netlink_dump+0x3ae/0x7e0 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f
4 locks held by dumper/549: ... #1: (rcu_read_lock){....}-{1:3}, at: inet6_dump_fib+0x88/0x620
#2: (&tb->tb6_lock){+.-.}-{3:3}, at: fib6_dump_table+0x1e9/0x360
#3: (rcu_read_lock){....}-{1:3}, at: rt6_dump_route+0x483/0x5f0
value changed: 0xffff8a7c44e05700 -> 0xffff8a7c45d60100
Reported by Kernel Concurrency Sanitizer on: CPU: 14 UID: 0 PID: 549 Comm: dumper Not tainted 7.2.0-rc7-virtme #38 PREEMPT(lazy) ...
Use hlist_for_each_entry_rcu() to safely iterate over the exception list.
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Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel contains a concurrency vulnerability within the IPv6 routing subsystem, specifically in the function rt6_nh_dump_exceptions which is responsible for iterating over route exceptions during network information dumps. The core technical flaw stems from an improper synchronization mechanism when accessing data structures that are protected by Read-Copy-Update (RCU) semantics but also subject to concurrent modifications via a separate spinlock. While the caller of this function correctly acquires the rcu_read_lock, which prevents memory reclamation and ensures visibility of existing pointers during traversal, it fails to acquire rt6_exception_lock. This lock is required by rt6_insert_exception when adding new entries to the exception list using hlist_add_head_rcu. The absence of the spinlock in the reading path creates a classic data race condition where one CPU core may be modifying the linked list structure while another CPU core traverses it, leading to potential memory corruption or kernel instability.
This vulnerability is classified under CWE-362, which denotes concurrent execution using shared resources with improper synchronization. The specific nature of this bug involves a read-write race on a shared data structure where the write operation modifies pointers within an hlist while the read operation dereferences those same pointers without holding the necessary exclusion lock. Although RCU provides mechanisms for safe reading during updates, it does not inherently protect against structural changes like insertion or deletion unless combined with appropriate locking strategies in this specific context. The Kernel Concurrency Sanitizer (KCSAN) detected this issue by observing a write to memory address 0xffff8a7c44c59620 on one CPU core during an ICMPv6 Path MTU update, while another CPU core performed a read from the same location during a fib6_dump_table operation triggered by a netlink dump request. This race condition can result in undefined behavior, including kernel panics, data corruption of routing tables, or information leaks if corrupted pointers are dereferenced later.
From an operational perspective, this vulnerability affects systems that actively manage IPv6 routes and exceptions, particularly those with high rates of Path MTU discovery updates combined with frequent route table dumps via netlink interfaces such as iproute2 tools. An attacker who can trigger rapid PMTU changes while simultaneously requesting detailed routing information could potentially exploit the race condition to crash the system or destabilize network connectivity. The impact is primarily denial-of-service through kernel instability, though in complex scenarios involving memory corruption, it might lead to privilege escalation if an attacker can control the contents of the corrupted data structure. This aligns with ATT&CK technique T1498, Network Denial of Service, as well as potential lateral movement implications if system stability is compromised during critical network operations.
The resolution involves replacing the standard hlist_for_each_entry macro with its RCU-safe counterpart, hlist_for_each_entry_rcu, within rt6_nh_dump_exceptions. This change ensures that the iteration logic properly respects the RCU read-side critical section boundaries and aligns with kernel best practices for traversing lists protected by both RCU and spinlocks in mixed-access scenarios. To mitigate this vulnerability on systems where patching is not immediately feasible, administrators should consider limiting the frequency of netlink route dumps during periods of high network churn or PMTU updates. Additionally, ensuring that all IPv6 routing operations are performed with up-to-date kernel versions containing this fix is critical for maintaining system integrity and preventing potential exploitation through concurrent race conditions in the networking stack.