netdev - [Bug] soft lockup in sys_sendmmsg in Linux kernel v6.14

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Message-ID: <CALm_T+2NP6RguKihvZbi_W61Yutup1ZcuNZBr950TJCGj-p8WQ@mail.gmail.com>
Date: Tue, 8 Jul 2025 15:49:57 +0800
From: Luka <luka.2016.cs@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, Kuniyuki Iwashima <kuniyu@...zon.com>, 
	Paolo Abeni <pabeni@...hat.com>, Willem de Bruijn <willemb@...gle.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>
Cc: Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [Bug] soft lockup in sys_sendmmsg in Linux kernel v6.14

Dear Linux Kernel Maintainers,

I hope this message finds you well.

I am writing to report a potential vulnerability I encountered during
testing of the Linux Kernel version v6.14.

Git Commit: 38fec10eb60d687e30c8c6b5420d86e8149f7557 (tag: v6.14)

Bug Location: __x64_sys_sendmmsg+0x9d/0x110 net/socket.c:2733

Bug report: https://pastebin.com/YrfrS22B

Entire kernel config: https://pastebin.com/MRWGr3nv

Root Cause Analysis:

A soft lockup occurs during invocation of the sys_sendmmsg() syscall
due to prolonged CPU occupation in packet transmission routines,
likely caused by excessive memory allocation or reclaim activity in
UDP socket send path. The issue involves __ip_append_data() and
udp_sendmsg() functions exhausting CPU cycles within the softirq
context, without yielding, which starves other kernel tasks and
triggers a watchdog timeout.

At present, I have not yet obtained a minimal reproducer for this
issue. However, I am actively working on reproducing it, and I will
promptly share any additional findings or a working reproducer as soon
as it becomes available.

Thank you very much for your time and attention to this matter. I
truly appreciate the efforts of the Linux kernel community.

Best regards,
Luka