CVE-2024-56655 in Linux
Summary
by MITRE • 12/27/2024
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_tables: do not defer rule destruction via call_rcu
nf_tables_chain_destroy can sleep, it can't be used from call_rcu callbacks.
Moreover, nf_tables_rule_release() is only safe for error unwinding, while transaction mutex is held and the to-be-desroyed rule was not exposed to either dataplane or dumps, as it deactives+frees without the required synchronize_rcu() in-between.
nft_rule_expr_deactivate() callbacks will change ->use counters of other chains/sets, see e.g. nft_lookup .deactivate callback, these must be serialized via transaction mutex.
Also add a few lockdep asserts to make this more explicit.
Calling synchronize_rcu() isn't ideal, but fixing this without is hard and way more intrusive. As-is, we can get:
WARNING: .. net/netfilter/nf_tables_api.c:5515 nft_set_destroy+0x.. Workqueue: events nf_tables_trans_destroy_work RIP: 0010:nft_set_destroy+0x3fe/0x5c0 Call Trace: nf_tables_trans_destroy_work+0x6b7/0xad0 process_one_work+0x64a/0xce0 worker_thread+0x613/0x10d0
In case the synchronize_rcu becomes an issue, we can explore alternatives.
One way would be to allocate nft_trans_rule objects + one nft_trans_chain object, deactivate the rules + the chain and then defer the freeing to the nft destroy workqueue. We'd still need to keep the synchronize_rcu path as a fallback to handle -ENOMEM corner cases though.
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Analysis
by VulDB Data Team • 01/12/2026
The vulnerability identified as CVE-2024-56655 resides within the Linux kernel's netfilter subsystem, specifically in the nf_tables component that manages packet filtering and manipulation rules. This issue represents a critical flaw in the kernel's memory management and synchronization mechanisms that could potentially lead to system instability or security compromise. The vulnerability stems from improper handling of rule destruction within the netfilter framework, where the kernel fails to properly coordinate between different synchronization primitives during rule cleanup operations.
The technical root cause involves a fundamental mismatch in how rule destruction is handled within the nf_tables subsystem. The function nf_tables_chain_destroy is capable of sleeping during execution, which makes it incompatible with call_rcu callback contexts where such sleeping behavior would cause system deadlocks or undefined behavior. Additionally, the nf_tables_rule_release function, which is intended for error unwinding scenarios, operates under specific constraints where the transaction mutex is held and the rule being destroyed has not yet been exposed to the dataplane or dumps. This creates a dangerous race condition where the rule destruction process bypasses proper synchronization mechanisms required for safe memory deallocation.
The vulnerability manifests through the improper handling of nft_rule_expr_deactivate() callbacks, which modify use counters of other chains and sets within the netfilter framework. These callbacks, particularly those involving nft_lookup.deactivate operations, must be serialized through the transaction mutex to prevent concurrent access issues. However, the current implementation fails to enforce this serialization, creating potential for data corruption or inconsistent state management. The lockdep assertions added to the codebase serve to make these synchronization requirements more explicit, but they cannot prevent the underlying architectural flaw.
The operational impact of this vulnerability extends beyond simple memory management issues to potentially affect system stability and security. When the kernel attempts to destroy rules, it triggers a workqueue execution path that can lead to warning messages indicating improper synchronization. The call trace shows execution flowing through nf_tables_trans_destroy_work, process_one_work, and worker_thread functions, indicating that the system is attempting to perform rule destruction operations in a context where proper synchronization has not been maintained. This could result in memory corruption, system crashes, or potentially exploitable conditions where attackers might manipulate the timing and execution of rule destruction to cause system instability.
The mitigation approach for this vulnerability involves implementing proper synchronization mechanisms to ensure that rule destruction operations occur in the correct context. While the immediate solution requires calling synchronize_rcu() to ensure proper memory barrier enforcement, this approach has performance implications that could become problematic in high-throughput networking environments. Alternative approaches include allocating dedicated nft_trans_rule and nft_trans_chain objects to manage the deactivation and freeing process more carefully, though these solutions would require significant code changes and could introduce their own complexity. The current implementation maintains the synchronize_rcu path as a fallback mechanism to handle memory allocation failures, ensuring that even in -ENOMEM corner cases, the system can maintain proper state consistency. This vulnerability aligns with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and could potentially be leveraged through ATT&CK techniques related to system compromise and privilege escalation.