Bug 2348553 (CVE-2025-21767)

Summary: CVE-2025-21767 kernel: clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: dfreiber, drow, jburrell, vkumar
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A vulnerability was found in the Linux kernel's clocksource subsystem, specifically affecting systems running with the PREEMPT_RT (Real-Time) patch. In real-time kernels, certain locks used within the random number generation functions, are implemented as sleeping locks. Calling these functions in an atomic context—where sleeping is not allowed—can lead to kernel warnings or crashes. The issue occurs when the get_random_u32() function is called within an atomic context, which is not permissible in real-time kernels due to the use of sleeping locks in the random number generation code.
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Description OSIDB Bzimport 2025-02-27 03:02:59 UTC 
In the Linux kernel, the following vulnerability has been resolved:

clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

The following bug report happened with a PREEMPT_RT kernel:

  BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog
  preempt_count: 1, expected: 0
  RCU nest depth: 0, expected: 0
  get_random_u32+0x4f/0x110
  clocksource_verify_choose_cpus+0xab/0x1a0
  clocksource_verify_percpu.part.0+0x6b/0x330
  clocksource_watchdog_kthread+0x193/0x1a0

It is due to the fact that clocksource_verify_choose_cpus() is invoked with
preemption disabled.  This function invokes get_random_u32() to obtain
random numbers for choosing CPUs.  The batched_entropy_32 local lock and/or
the base_crng.lock spinlock in driver/char/random.c will be acquired during
the call. In PREEMPT_RT kernel, they are both sleeping locks and so cannot
be acquired in atomic context.

Fix this problem by using migrate_disable() to allow smp_processor_id() to
be reliably used without introducing atomic context. preempt_disable() is
then called after clocksource_verify_choose_cpus() but before the
clocksource measurement is being run to avoid introducing unexpected
latency.
Comment 1 Michal Findra 2025-02-27 17:53:16 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025022605-CVE-2025-21767-4c7f@gregkh/T