CVE-2025-21720 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
xfrm: delete intermediate secpath entry in packet offload mode
Packets handled by hardware have added secpath as a way to inform XFRM core code that this path was already handled. That secpath is not needed at all after policy is checked and it is removed later in the stack.
However, in the case of IP forwarding is enabled (/proc/sys/net/ipv4/ip_forward), that secpath is not removed and packets which already were handled are reentered to the driver TX path with xfrm_offload set.
The following kernel panic is observed in mlx5 in such case:
mlx5_core 0000:04:00.0 enp4s0f0np0: Link up mlx5_core 0000:04:00.1 enp4s0f1np1: Link up Initializing XFRM netlink socket IPsec XFRM device driver BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD 0 P4D 0 Oops: Oops: 0010 [#1] PREEMPT SMP
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.13.0-rc1-alex #3 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014 RIP: 0010:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. RSP: 0018:ffffb87380003800 EFLAGS: 00010206 RAX: ffff8df004e02600 RBX: ffffb873800038d8 RCX: 00000000ffff98cf RDX: ffff8df00733e108 RSI: ffff8df00521fb80 RDI: ffff8df001661f00 RBP: ffffb87380003850 R08: ffff8df013980000 R09: 0000000000000010 R10: 0000000000000002 R11: 0000000000000002 R12: ffff8df001661f00 R13: ffff8df00521fb80 R14: ffff8df00733e108 R15: ffff8df011faf04e FS: 0000000000000000(0000) GS:ffff8df46b800000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: ffffffffffffffd6 CR3: 0000000106384000 CR4: 0000000000350ef0 Call Trace: ? show_regs+0x63/0x70 ? __die_body+0x20/0x60 ? __die+0x2b/0x40 ? page_fault_oops+0x15c/0x550 ? do_user_addr_fault+0x3ed/0x870 ? exc_page_fault+0x7f/0x190 ? asm_exc_page_fault+0x27/0x30 mlx5e_ipsec_handle_tx_skb+0xe7/0x2f0 [mlx5_core]
mlx5e_xmit+0x58e/0x1980 [mlx5_core]
? __fib_lookup+0x6a/0xb0 dev_hard_start_xmit+0x82/0x1d0 sch_direct_xmit+0xfe/0x390 __dev_queue_xmit+0x6d8/0xee0 ? __fib_lookup+0x6a/0xb0 ? internal_add_timer+0x48/0x70 ? mod_timer+0xe2/0x2b0 neigh_resolve_output+0x115/0x1b0 __neigh_update+0x26a/0xc50 neigh_update+0x14/0x20 arp_process+0x2cb/0x8e0 ? __napi_build_skb+0x5e/0x70 arp_rcv+0x11e/0x1c0 ? dev_gro_receive+0x574/0x820 __netif_receive_skb_list_core+0x1cf/0x1f0 netif_receive_skb_list_internal+0x183/0x2a0 napi_complete_done+0x76/0x1c0 mlx5e_napi_poll+0x234/0x7a0 [mlx5_core]
__napi_poll+0x2d/0x1f0 net_rx_action+0x1a6/0x370 ? atomic_notifier_call_chain+0x3b/0x50 ? irq_int_handler+0x15/0x20 [mlx5_core]
handle_softirqs+0xb9/0x2f0 ? handle_irq_event+0x44/0x60 irq_exit_rcu+0xdb/0x100 common_interrupt+0x98/0xc0 asm_common_interrupt+0x27/0x40 RIP: 0010:pv_native_safe_halt+0xb/0x10 Code: 09 c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 0f 22 0f 1f 84 00 00 00 00 00 90 eb 07 0f 00 2d 7f e9 36 00 fb 40 00 83 ff 07 77 21 89 ff ff 24 fd 88 3d a1 bd 0f 21 f8 RSP: 0018:ffffffffbe603de8 EFLAGS: 00000202 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000f92f46680 RDX: 0000000000000037 RSI: 00000000ffffffff RDI: 00000000000518d4 RBP: ffffffffbe603df0 R08: 000000cd42e4dffb R09: ffffffffbe603d70 R10: 0000004d80d62680 R11: 0000000000000001 R12: ffffffffbe60bf40 R13: 0000000000000000 R14: 0000000000000000 R15: ffffffffbe60aff8 ? default_idle+0x9/0x20 arch_cpu_idle+0x9/0x10 default_idle_call+0x29/0xf0 do_idle+0x1f2/0x240 cpu_startup_entry+0x2c/0x30 rest_init+0xe7/0x100 start_kernel+0x76b/0xb90 x86_64_start_reservations+0x18/0x30 x86_64_start_kernel+0xc0/0x110 ? setup_ghcb+0xe/0x130 common_startup_64+0x13e/0x141 Modules linked in: esp4_offload esp4 xfrm_interface xfrm6_tunnel tunnel4 tunnel6 xfrm_user xfrm_algo binf ---truncated---
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21720 resides within the Linux kernel's implementation of the Extended Security Management (XFRM) subsystem, specifically in how it handles packet processing when hardware offload is enabled. This issue manifests when IP forwarding is enabled, causing a kernel panic due to a NULL pointer dereference in the mlx5 network driver. The root cause lies in the improper handling of secpath entries during packet processing, particularly in the context of hardware offload operations where packets are already processed by the XFRM core but are not correctly cleaned up before being re-entered into the hardware transmit path.
The technical flaw occurs because when IP forwarding is enabled, secpath entries that were initially added to inform the XFRM core that a packet path was already handled are not being removed. These entries persist in the packet metadata and are subsequently reprocessed by the network driver's transmit path with xfrm_offload flag set. This leads to a scenario where the driver attempts to access memory locations that have been freed or are invalid, resulting in a kernel NULL pointer dereference. The vulnerability is particularly impactful when using Mellanox mlx5 network adapters, as demonstrated by the crash trace showing mlx5e_ipsec_handle_tx_skb as the failing function.
The operational impact of this vulnerability is significant as it can cause complete system crashes or panics, effectively rendering the affected system unavailable. The issue affects systems running Linux kernel versions where the XFRM subsystem handles hardware offload for IPsec packets, particularly when IP forwarding is enabled. This makes the vulnerability exploitable in environments where IP forwarding is required for network routing, such as routers or network appliances that use IPsec for secure communications. The panic occurs during the normal packet processing flow, meaning any traffic passing through the affected system can trigger the vulnerability, potentially leading to denial of service attacks or system instability.
Mitigation strategies for this vulnerability include disabling IP forwarding if it is not required for the system's operation, which prevents the problematic code path from being executed. Additionally, applying the kernel patch that resolves the issue by ensuring proper cleanup of secpath entries in hardware offload mode is the recommended approach. Organizations should prioritize updating to kernel versions that contain the fix, which typically involves modifications to how the XFRM subsystem handles packet metadata in offload scenarios. The fix aligns with CWE-476, which addresses NULL pointer dereference vulnerabilities, and follows ATT&CK techniques related to privilege escalation through kernel exploitation. Network administrators should also monitor for unusual packet processing behavior or system panics that may indicate exploitation attempts.
This vulnerability highlights the complexity of kernel security when dealing with hardware offload mechanisms and the need for careful handling of packet metadata across different processing paths. The interaction between XFRM, IP forwarding, and hardware offload creates a specific edge case that requires careful consideration in kernel design and testing. The issue underscores the importance of thorough testing in environments where multiple network processing mechanisms intersect, particularly in high-performance networking scenarios where hardware acceleration is utilized.