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 napis 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 napis weight always non-zero solves this problem.

Triggering this issue requires system-wide admin (setting is not namespaced).

Affected Software

References

• 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