Subject: CVE-2024-50032: rcu/nocb: Fix rcuog wake-up from offline softirq

From: Greg Kroah-Hartman <[email-masked]>

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

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

rcu/nocb: Fix rcuog wake-up from offline softirq

After a CPU has set itself offline and before it eventually calls
rcutree_report_cpu_dead(), there are still opportunities for callbacks
to be enqueued, for example from a softirq. When that happens on NOCB,
the rcuog wake-up is deferred through an IPI to an online CPU in order
not to call into the scheduler and risk arming the RT-bandwidth after
hrtimers have been migrated out and disabled.

But performing a synchronized IPI from a softirq is buggy as reported in
the following scenario:

        WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single
        Modules linked in: rcutorture torture
        CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1
        Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120
        RIP: 0010:smp_call_function_single
        <IRQ>
        swake_up_one_online
        __call_rcu_nocb_wake
        __call_rcu_common
        ? rcu_torture_one_read
        call_timer_fn
        __run_timers
        run_timer_softirq
        handle_softirqs
        irq_exit_rcu
        ? tick_handle_periodic
        sysvec_apic_timer_interrupt
        </IRQ>

Fix this with forcing deferred rcuog wake up through the NOCB timer when
the CPU is offline. The actual wake up will happen from
rcutree_report_cpu_dead().

The Linux kernel CVE team has assigned CVE-2024-50032 to this issue.

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

	Issue introduced in 6.6.54 with commit 9b52ee18f6d2 and fixed in 6.6.57 with commit 84a5feebba10
	Issue introduced in 6.11.2 with commit 00a611229bfa and fixed in 6.11.4 with commit e66b1e01f2eb
	Issue introduced in 6.12-rc1 with commit 9139f93209d1 and fixed in 6.12-rc3 with commit f7345ccc62a4
	Issue introduced in 6.1.113 with commit ec2777e57f2f
	Issue introduced in 6.10.13 with commit bbeaae6f79b2

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-2024-50032
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:
	kernel/rcu/tree_nocb.h

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/84a5feebba10354c683983f5f1372a144225e4c2
	https://git.kernel.org/stable/c/e66b1e01f2eb3209d08122572f41f7838b79540d
	https://git.kernel.org/stable/c/f7345ccc62a4b880cf76458db5f320725f28e400