CVE-2025-21806: linux kernel — Fix

Description

In the Linux kernel, the following vulnerability has been resolved: net: let net.core.dev_weight always be non-zero The following problem was encountered during stability test: (NULL net_device): NAPI poll function process_backlog+0x0/0x530 \ returned 1, exceeding its budget of 0. ------------[ cut here ]------------ list_add double add: new=ffff88905f746f48, prev=ffff88905f746f48, \ next=ffff88905f746e40. WARNING: CPU: 18 PID: 5462 at lib/list_debug.c:35 \ __list_add_valid_or_report+0xf3/0x130 CPU: 18 UID: 0 PID: 5462 Comm: ping Kdump: loaded Not tainted 6.13.0-rc7+ RIP: 0010:__list_add_valid_or_report+0xf3/0x130 Call Trace: ? __warn+0xcd/0x250 ? __list_add_valid_or_report+0xf3/0x130 enqueue_to_backlog+0x923/0x1070 netif_rx_internal+0x92/0x2b0 __netif_rx+0x15/0x170 loopback_xmit+0x2ef/0x450 dev_hard_start_xmit+0x103/0x490 __dev_queue_xmit+0xeac/0x1950 ip_finish_output2+0x6cc/0x1620 ip_output+0x161/0x270 ip_push_pending_frames+0x155/0x1a0 raw_sendmsg+0xe13/0x1550 __sys_sendto+0x3bf/0x4e0 __x64_sys_sendto+0xdc/0x1b0 do_syscall_64+0x5b/0x170 entry_SYSCALL_64_after_hwframe+0x76/0x7e The reproduction command is as follows: sysctl -w net.core.dev_weight=0 ping 127.0.0.1 This is because when the napi's weight is set to 0, process_backlog() may return 0 and clear the NAPI_STATE_SCHED bit of napi->state, causing this napi to be re-polled in net_rx_action() until __do_softirq() times out. Since the NAPI_STATE_SCHED bit has been cleared, napi_schedule_rps() can be retriggered in enqueue_to_backlog(), causing this issue. Making the napi's weight always non-zero solves this problem. Triggering this issue requires system-wide admin (setting is not namespaced).

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CVSS v3.1 Score

5.5

MEDIUM

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected Products

Vendor	Product	Versions
linux	linux_kernel	2.6.13 — 5.4.291
linux	linux_kernel	5.5 — 5.10.235
linux	linux_kernel	5.11 — 5.15.179
linux	linux_kernel	5.16 — 6.1.129
linux	linux_kernel	6.2 — 6.6.76
linux	linux_kernel	6.7 — 6.12.13
linux	linux_kernel	6.13 — 6.13.2
linux	linux_kernel	All

References

Advisories & Patches

https://git.kernel.org/stable/c/0e2f1d93d287d544d26f8ff293ea820a8079b9f8 https://git.kernel.org/stable/c/1489824e5226a26841c70639ebd2d1aed390764b https://git.kernel.org/stable/c/33e2168788f8fb5cb8bd4f36cb1ef37d1d34dada https://git.kernel.org/stable/c/5860abbf15eeb61838b5e32e721ba67b0aa84450 https://git.kernel.org/stable/c/6ce38b5a6a49e65bad163162a54cb3f104c40b48 https://git.kernel.org/stable/c/c337c08819a4ec49edfdcd8fc46fbee120d8a5b2 https://git.kernel.org/stable/c/d0e0f9c8218826926d7692980c98236d9f21fd3c https://git.kernel.org/stable/c/d1f9f79fa2af8e3b45cffdeef66e05833480148a

Other References

https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html https://cert-portal.siemens.com/productcert/html/ssa-265688.html https://cert-portal.siemens.com/productcert/html/ssa-503939.html

Frequently Asked Questions

What is CVE-2025-21806? +

In the Linux kernel, the following vulnerability has been resolved: net: let net.core.dev_weight always be non-zero The following problem was encountered during stability test: (NULL net_device): NAPI poll function process_backlog+0x0/0x530 \ returned 1, exceeding its budget of 0. ------------[ cut here ]------------ list_add double add: new=ffff88905f746f48, prev=ffff88905f746f48, \ next=ffff88905f746e40. WARNING: CPU: 18 PID: 5462 at lib/list_debug.c:35 \ __list_add_valid_or_report+0xf3/0x130 CPU: 18 UID: 0 PID: 5462 Comm: ping Kdump: loaded Not tainted 6.13.0-rc7+ RIP: 0010:__list_add_valid_or_report+0xf3/0x130 Call Trace: ? __warn+0xcd/0x250 ? __list_add_valid_or_report+0xf3/0x130 enqueue_to_backlog+0x923/0x1070 netif_rx_internal+0x92/0x2b0 __netif_rx+0x15/0x170 loopback_xmit+0x2ef/0x450 dev_hard_start_xmit+0x103/0x490 __dev_queue_xmit+0xeac/0x1950 ip_finish_output2+0x6cc/0x1620 ip_output+0x161/0x270 ip_push_pending_frames+0x155/0x1a0 raw_sendmsg+0xe13/0x1550 __sys_sendto+0x3bf/0x4e0 __x64_sys_sendto+0xdc/0x1b0 do_syscall_64+0x5b/0x170 entry_SYSCALL_64_after_hwframe+0x76/0x7e The reproduction command is as follows: sysctl -w net.core.dev_weight=0 ping 127.0.0.1 This is because when the napi's weight is set to 0, process_backlog() may return 0 and clear the NAPI_STATE_SCHED bit of napi->state, causing this napi to be re-polled in net_rx_action() until __do_softirq() times out. Since the NAPI_STATE_SCHED bit has been cleared, napi_schedule_rps() can be retriggered in enqueue_to_backlog(), causing this issue. Making the napi's weight always non-zero solves this problem. Triggering this issue requires system-wide admin (setting is not namespaced). It has a CVSS v3.1 base score of 5.5 (MEDIUM).

How severe is CVE-2025-21806? +

CVE-2025-21806 has a CVSS v3.1 score of 5.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-21806? +

CVE-2025-21806 affects products from linux, specifically: linux_kernel. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-21806? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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