rcu: Protect ->defer_qs_iw_pending from data race

Description

In the Linux kernel, the following vulnerability has been resolved: rcu: Protect ->defer_qs_iw_pending from data race On kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() is invoked within an interrupts-disabled region of code [1], it will invoke rcu_read_unlock_special(), which uses an irq-work handler to force the system to notice when the RCU read-side critical section actually ends. That end won't happen until interrupts are enabled at the soonest. In some kernels, such as those booted with rcutree.use_softirq=y, the irq-work handler is used unconditionally. The per-CPU rcu_data structure's ->defer_qs_iw_pending field is updated by the irq-work handler and is both read and updated by rcu_read_unlock_special(). This resulted in the following KCSAN splat: ------------------------------------------------------------------------ BUG: KCSAN: data-race in rcu_preempt_deferred_qs_handler / rcu_read_unlock_special read to 0xffff96b95f42d8d8 of 1 bytes by task 90 on cpu 8: rcu_read_unlock_special+0x175/0x260 __rcu_read_unlock+0x92/0xa0 rt_spin_unlock+0x9b/0xc0 __local_bh_enable+0x10d/0x170 __local_bh_enable_ip+0xfb/0x150 rcu_do_batch+0x595/0xc40 rcu_cpu_kthread+0x4e9/0x830 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30 write to 0xffff96b95f42d8d8 of 1 bytes by task 88 on cpu 8: rcu_preempt_deferred_qs_handler+0x1e/0x30 irq_work_single+0xaf/0x160 run_irq_workd+0x91/0xc0 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30 no locks held by irq_work/8/88. irq event stamp: 200272 hardirqs last enabled at (200272): [<ffffffffb0f56121>] finish_task_switch+0x131/0x320 hardirqs last disabled at (200271): [<ffffffffb25c7859>] __schedule+0x129/0xd70 softirqs last enabled at (0): [<ffffffffb0ee093f>] copy_process+0x4df/0x1cc0 softirqs last disabled at (0): [<0000000000000000>] 0x0 ------------------------------------------------------------------------ The problem is that irq-work handlers run with interrupts enabled, which means that rcu_preempt_deferred_qs_handler() could be interrupted, and that interrupt handler might contain an RCU read-side critical section, which might invoke rcu_read_unlock_special(). In the strict KCSAN mode of operation used by RCU, this constitutes a data race on the ->defer_qs_iw_pending field. This commit therefore disables interrupts across the portion of the rcu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pending field. This suffices because this handler is not a fast path.

Timeline

Scores

EPSS (exploitation in 30 days)

0.18% · top 92% of all CVEs (12 Nov 2026)

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.0.

Affected products

• Linux · Linux

  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < 74f58f382a7c8333f8d09701aefaa25913bdbe0e
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < f937759c7432d6151b73e1393b6517661813d506
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < 0ad84d62217488e679ecc90e8628980dcc003de3
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < b5de8d80b5d049f051b95d9b1ee50ae4ab656124
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < b55947b725f190396f475d5d0c59aa855a4d8895
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < e35e711c78c8a4c43330c0dcb1c4d507a19c20f4
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < 90de9c94ea72327cfa9c2c9f6113c23a513af60b
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < 55e11f6776798b27cf09a7aa0d718415d4fc9cf5
  • affected 0864f057b050bc6dd68106b3185e02db5140012d – < 90c09d57caeca94e6f3f87c49e96a91edd40cbfd

• Linux · Linux

  • affected 5.3
  • unaffected < 5.3
  • unaffected 5.4.297 – ≤ 5.4.*
  • unaffected 5.10.241 – ≤ 5.10.*
  • unaffected 5.15.190 – ≤ 5.15.*
  • unaffected 6.1.149 – ≤ 6.1.*
  • unaffected 6.6.103 – ≤ 6.6.*
  • unaffected 6.12.43 – ≤ 6.12.*
  • unaffected 6.15.11 – ≤ 6.15.*
  • unaffected 6.16.2 – ≤ 6.16.*
  • unaffected 6.17 – ≤ *

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• EUVD EUVD-2025-28962 (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2025-39749](https://cveledger.app/badge/CVE-2025-39749.svg)](https://cveledger.app/cve/CVE-2025-39749)

Record last updated 12 May 2026, 12:06 UTC.