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.
Affected products
Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.
Linux Linux
Default version status: unaffected
0864f057b050bc6dd68106b3185e02db5140012d to < 74f58f382a7c8333f8d09701aefaa25913bdbe0eaffected0864f057b050bc6dd68106b3185e02db5140012d to < f937759c7432d6151b73e1393b6517661813d506affected0864f057b050bc6dd68106b3185e02db5140012d to < 0ad84d62217488e679ecc90e8628980dcc003de3affected0864f057b050bc6dd68106b3185e02db5140012d to < b5de8d80b5d049f051b95d9b1ee50ae4ab656124affected0864f057b050bc6dd68106b3185e02db5140012d to < b55947b725f190396f475d5d0c59aa855a4d8895affected0864f057b050bc6dd68106b3185e02db5140012d to < e35e711c78c8a4c43330c0dcb1c4d507a19c20f4affected0864f057b050bc6dd68106b3185e02db5140012d to < 90de9c94ea72327cfa9c2c9f6113c23a513af60baffected0864f057b050bc6dd68106b3185e02db5140012d to < 55e11f6776798b27cf09a7aa0d718415d4fc9cf5affected0864f057b050bc6dd68106b3185e02db5140012d to < 90c09d57caeca94e6f3f87c49e96a91edd40cbfdaffected
Linux Linux
Default version status: affected
5.3affected0 to < 5.3unaffected5.4.297 to ≤ 5.4.*unaffected5.10.241 to ≤ 5.10.*unaffected5.15.190 to ≤ 5.15.*unaffected6.1.149 to ≤ 6.1.*unaffected6.6.103 to ≤ 6.6.*unaffected6.12.43 to ≤ 6.12.*unaffected6.15.11 to ≤ 6.15.*unaffected6.16.2 to ≤ 6.16.*unaffected6.17 to ≤ *unaffected
References
- https://git.kernel.org/stable/c/74f58f382a7c8333f8d09701aefaa25913bdbe0e ↗
- https://git.kernel.org/stable/c/f937759c7432d6151b73e1393b6517661813d506 ↗
- https://git.kernel.org/stable/c/0ad84d62217488e679ecc90e8628980dcc003de3 ↗
- https://git.kernel.org/stable/c/b5de8d80b5d049f051b95d9b1ee50ae4ab656124 ↗
- https://git.kernel.org/stable/c/b55947b725f190396f475d5d0c59aa855a4d8895 ↗
- https://git.kernel.org/stable/c/e35e711c78c8a4c43330c0dcb1c4d507a19c20f4 ↗
- https://git.kernel.org/stable/c/90de9c94ea72327cfa9c2c9f6113c23a513af60b ↗
- https://git.kernel.org/stable/c/55e11f6776798b27cf09a7aa0d718415d4fc9cf5 ↗
- https://git.kernel.org/stable/c/90c09d57caeca94e6f3f87c49e96a91edd40cbfd ↗
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html ↗
- https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html ↗
- https://cert-portal.siemens.com/productcert/html/ssa-032379.html ↗