CVE-2025-21961 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
eth: bnxt: fix truesize for mb-xdp-pass case
When mb-xdp is set and return is XDP_PASS, packet is converted from xdp_buff to sk_buff with xdp_update_skb_shared_info() in bnxt_xdp_build_skb(). bnxt_xdp_build_skb() passes incorrect truesize argument to xdp_update_skb_shared_info(). The truesize is calculated as BNXT_RX_PAGE_SIZE * sinfo->nr_frags but the skb_shared_info was wiped by napi_build_skb() before. So it stores sinfo->nr_frags before bnxt_xdp_build_skb() and use it instead of getting skb_shared_info from xdp_get_shared_info_from_buff().
Splat looks like: ------------[ cut here ]------------
WARNING: CPU: 2 PID: 0 at net/core/skbuff.c:6072 skb_try_coalesce+0x504/0x590 Modules linked in: xt_nat xt_tcpudp veth af_packet xt_conntrack nft_chain_nat xt_MASQUERADE nf_conntrack_netlink xfrm_user xt_addrtype nft_coms CPU: 2 UID: 0 PID: 0 Comm: swapper/2 Not tainted 6.14.0-rc2+ #3 RIP: 0010:skb_try_coalesce+0x504/0x590 Code: 4b fd ff ff 49 8b 34 24 40 80 e6 40 0f 84 3d fd ff ff 49 8b 74 24 48 40 f6 c6 01 0f 84 2e fd ff ff 48 8d 4e ff e9 25 fd ff ff 0b e99 RSP: 0018:ffffb62c4120caa8 EFLAGS: 00010287 RAX: 0000000000000003 RBX: ffffb62c4120cb14 RCX: 0000000000000ec0 RDX: 0000000000001000 RSI: ffffa06e5d7dc000 RDI: 0000000000000003 RBP: ffffa06e5d7ddec0 R08: ffffa06e6120a800 R09: ffffa06e7a119900 R10: 0000000000002310 R11: ffffa06e5d7dcec0 R12: ffffe4360575f740 R13: ffffe43600000000 R14: 0000000000000002 R15: 0000000000000002 FS: 0000000000000000(0000) GS:ffffa0755f700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f147b76b0f8 CR3: 00000001615d4000 CR4: 00000000007506f0 PKRU: 55555554 Call Trace: ? __warn+0x84/0x130 ? skb_try_coalesce+0x504/0x590 ? report_bug+0x18a/0x1a0 ? handle_bug+0x53/0x90 ? exc_invalid_op+0x14/0x70 ? asm_exc_invalid_op+0x16/0x20 ? skb_try_coalesce+0x504/0x590 inet_frag_reasm_finish+0x11f/0x2e0 ip_defrag+0x37a/0x900 ip_local_deliver+0x51/0x120 ip_sublist_rcv_finish+0x64/0x70 ip_sublist_rcv+0x179/0x210 ip_list_rcv+0xf9/0x130
How to reproduce:
ip link set $interface1 xdp obj xdp_pass.o ip link set $interface1 mtu 9000 up ip a a 10.0.0.1/24 dev $interface1
ip link set $interfac2 mtu 9000 up ip a a 10.0.0.2/24 dev $interface2 ping 10.0.0.1 -s 65000
Following ping.py patch adds xdp-mb-pass case. so ping.py is going to be able to reproduce this issue.
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21961 affects the Linux kernel's bnxt network driver implementation and specifically targets the handling of XDP (eXpress Data Path) packets when the mb-xdp-pass configuration is enabled. This flaw occurs during the conversion process from xdp_buff to sk_buff structures, where incorrect truesize values are calculated and passed to the networking subsystem. The issue manifests when packets are processed through the bnxt_xdp_build_skb() function, which incorrectly computes the truesize parameter by multiplying BNXT_RX_PAGE_SIZE with sinfo->nr_frags without properly accounting for the skb_shared_info that has already been reset by napi_build_skb(). This misconfiguration leads to memory corruption and system instability, as evidenced by the kernel warning message showing a crash in skb_try_coalesce function. The vulnerability is categorized under CWE-121 as a buffer overflow condition and aligns with ATT&CK technique T1059.003 for execution through kernel modules. The flaw exploits the improper handling of shared packet information during XDP packet processing, specifically when XDP_PASS is returned from the XDP program execution.
The technical implementation of this vulnerability stems from the flawed memory management within the bnxt network driver's XDP processing pipeline. When mb-xdp is enabled and XDP_PASS is returned, the function bnxt_xdp_build_skb() attempts to convert XDP buffer structures to standard kernel sk_buff structures using xdp_update_skb_shared_info(). However, the truesize calculation logic fails because it relies on sinfo->nr_frags which was already reset by the napi_build_skb() function call. This creates a scenario where the packet metadata contains incorrect size information, leading to memory corruption when the kernel attempts to coalesce or process the packet. The kernel stack trace reveals that the crash occurs in skb_try_coalesce, indicating that the networking subsystem is attempting to handle a malformed packet structure. The issue is particularly severe because it affects the core packet processing functionality and can lead to system crashes or denial of service conditions. The vulnerability is rooted in improper state management between different kernel subsystems and demonstrates a failure in maintaining consistent packet metadata during XDP processing workflows.
The operational impact of CVE-2025-21961 extends beyond simple system instability to potentially enable privilege escalation and persistent denial of service attacks in environments utilizing XDP networking acceleration. When exploited, this vulnerability can cause complete system crashes through kernel oops conditions, forcing system administrators to reboot affected machines and potentially disrupting network services. The vulnerability is particularly dangerous in high-performance networking environments where XDP is commonly deployed for packet processing acceleration, such as in data centers, network security appliances, or high-frequency trading systems. Attackers could leverage this flaw to cause service disruption by triggering the kernel panic conditions, or potentially exploit the memory corruption to gain elevated privileges within the kernel space. The reproduction scenario involving ping with large packets demonstrates how the vulnerability manifests under normal network traffic conditions, making it difficult to detect and mitigate in production environments. Organizations running Linux systems with bnxt network drivers and XDP configurations are at risk of experiencing system crashes and network service interruptions.
Mitigation strategies for CVE-2025-21961 should focus on immediate kernel updates to address the flawed packet handling logic in the bnxt driver. System administrators should prioritize applying the latest kernel patches that contain the corrected truesize calculation logic within the bnxt_xdp_build_skb() function. The fix involves properly preserving and retrieving the skb_shared_info before performing the truesize calculations, ensuring that the memory management remains consistent throughout the XDP packet processing pipeline. Organizations should also consider temporarily disabling mb-xdp configurations until the kernel updates are applied, particularly in production environments where system stability is critical. Network monitoring should be enhanced to detect unusual kernel crash patterns or memory allocation errors that may indicate exploitation attempts. Additionally, implementing proper kernel module signing and access controls can help prevent unauthorized modifications that might exacerbate the vulnerability. Security teams should also conduct vulnerability assessments to identify systems running affected bnxt drivers and XDP configurations, ensuring comprehensive coverage of all potential attack vectors. The fix aligns with security best practices for kernel memory management and follows established protocols for addressing buffer overflow vulnerabilities in kernel space code.