CVE-2026-80739 in Linux
Summary
by MITRE • 09/03/2026
In the Linux kernel, the following vulnerability has been resolved:
net/mlx5e: TC, Check if flow is PEER before acquiring devcom lock
In case __mlx5e_add_fdb_flow() fails in lower levels, the flow is deleted via mlx5e_tc_del_flow(), and mlx5e_tc_del_flow() is acquiring ESW devcom lock without condition. In addition, in case of peer_flow, __mlx5e_add_fdb_flow() is called while holding ESW devcom comp lock. This results in an AA deadlock.
To fix this, introduce a new PEER flag that is set on flows created as peer flows (the duplicate flows on peer devices), and check it in mlx5e_tc_del_flow() before acquiring ESW devcom lock.
Lockdep splat: ============================================ WARNING: possible recursive locking detected ============================================ Possible unsafe locking scenario: CPU0 ---- lock(&comp->lock_key#2); lock(&comp->lock_key#2); *** DEADLOCK *** Call Trace: <TASK> dump_stack_lvl+0x69/0xa0 print_deadlock_bug.cold+0xbd/0xca __lock_acquire+0x1671/0x2ec0 lock_acquire+0x10e/0x2e0 down_read+0x95/0x430 mlx5_devcom_for_each_peer_begin+0x4e/0xe0 [mlx5_core]
mlx5e_tc_del_flow+0x11d/0xa70 [mlx5_core]
mlx5e_flow_put+0x99/0x100 [mlx5_core]
__mlx5e_add_fdb_flow+0x409/0xf00 [mlx5_core]
mlx5e_configure_flower+0x2a86/0x4100 [mlx5_core]
mlx5e_rep_setup_tc_cls_flower+0x12f/0x1b0 [mlx5_core]
mlx5e_rep_setup_tc_cb+0x153/0x750 [mlx5_core]
tc_setup_cb_add+0x1dc/0x470 fl_change+0x2f4d/0x626d [cls_flower]
tc_new_tfilter+0x79b/0x2310 rtnetlink_rcv_msg+0x778/0xad0 do_syscall_64+0x70/0x960 entry_SYSCALL_64_after_hwframe+0x4b/0x53 </TASK>
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Analysis
by VulDB Data Team • 09/03/2026
The Linux kernel networking subsystem for Mellanox ConnectX adapters, specifically the mlx5e driver handling Traffic Control (TC) flower classification, contained a critical concurrency flaw leading to an ABBA deadlock. This vulnerability arises from improper lock ordering and missing conditional checks when managing flow rules on peer devices within the devcom infrastructure. The core issue is located in the interaction between __mlx5e_add_fdb_flow() and mlx5e_tc_del_flow(). When adding a new forwarding database (FDB) flow, if lower-level initialization fails, the kernel attempts to clean up by deleting the partially created flow via mlx5e_tc_del_flow(). This deletion routine unconditionally acquires the ESW devcom lock. However, in scenarios involving peer flows—where duplicate rules are synchronized across multiple physical or virtual devices—the parent function __mlx5e_add_fdb_flow() may already be holding a related component lock (comp->lock) when it invokes the flow addition logic. Consequently, if an error occurs during this nested operation, the cleanup path attempts to acquire the devcom lock while still holding the comp lock, whereas other code paths might hold the devcom lock and attempt to acquire the comp lock later. This circular dependency creates a classic ABBA deadlock scenario where two CPU threads or execution contexts wait indefinitely for each other's locks, resulting in system hangs or kernel panics that require a reboot to resolve.
From a technical perspective, this vulnerability is classified under CWE-833: Deadlock and aligns with MITRE ATT&CK techniques related to resource exhaustion and denial of service through lock contention. The root cause is a failure to distinguish between flows created locally versus those propagated as peers during the error handling phase. By treating all flow deletions identically regardless of their origin, the driver violates the established locking hierarchy within the mlx5 core subsystem. The introduction of a PEER flag allows the deletion routine to identify when it is operating on a peer-derived flow that was initiated while holding higher-level locks. When this flag is set, the code path bypasses the acquisition of the ESW devcom lock during cleanup, thereby breaking the circular dependency chain. This fix ensures that lock acquisition respects the existing hold context, preventing the recursive locking detected by Lockdep and reported in kernel logs as a possible unsafe locking scenario involving comp->lock_key#2.
The operational impact of this vulnerability is significant for systems relying on complex traffic steering configurations with peer device synchronization. An attacker or automated process triggering rapid creation and failure of flower filter rules could induce repeated deadlocks, effectively denying service to network functions managed by the mlx5e driver. This includes features like hardware offloading, QoS policies, and firewall rules that depend on stable TC rule management. The deadlock manifests as a complete stall in the affected kernel thread or potentially system-wide if critical networking paths are blocked. Mitigation involves applying the upstream Linux kernel patch that introduces the PEER flag check within mlx5e_tc_del_flow(). Administrators should ensure their systems run patched versions of the mlx5_core and mlx5en modules. Additionally, monitoring for lockdep splats in dmesg output can serve as an early warning indicator for this specific race condition before it results in a full system hang.