linux-cve-announce - CVE-2025-39900: net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT

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Message-ID: <2025100117-CVE-2025-39900-bb5f@gregkh>
Date: Wed,  1 Oct 2025 09:43:24 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-39900: net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

syzbot reported a WARNING in est_timer() [1]

Problem here is that with CONFIG_PREEMPT_RT=y, timer callbacks
can be preempted.

Adopt preempt_disable_nested()/preempt_enable_nested() to fix this.

[1]
 WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 __seqprop_assert include/linux/seqlock.h:221 [inline]
 WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
 RIP: 0010:__seqprop_assert include/linux/seqlock.h:221 [inline]
 RIP: 0010:est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
Call Trace:
 <TASK>
  call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
  expire_timers kernel/time/timer.c:1798 [inline]
  __run_timers kernel/time/timer.c:2372 [inline]
  __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
  run_timer_base kernel/time/timer.c:2393 [inline]
  run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
  handle_softirqs+0x22c/0x710 kernel/softirq.c:579
  __do_softirq kernel/softirq.c:613 [inline]
  run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
  smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
  kthread+0x70e/0x8a0 kernel/kthread.c:463
  ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

The Linux kernel CVE team has assigned CVE-2025-39900 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.12.46 with commit a22ec2ee824be30803068a52f78f7ffe3bc879fb
	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.16.6 with commit e79923824c48b930609680be04cb29253fc4a17d
	Issue introduced in 6.12 with commit d2d6422f8bd17c6bb205133e290625a564194496 and fixed in 6.17 with commit 9f74c0ea9b26d1505d55b61e36b1623dd347e1d1

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-39900
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/core/gen_estimator.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a22ec2ee824be30803068a52f78f7ffe3bc879fb
	https://git.kernel.org/stable/c/e79923824c48b930609680be04cb29253fc4a17d
	https://git.kernel.org/stable/c/9f74c0ea9b26d1505d55b61e36b1623dd347e1d1