Subject: CVE-2024-35929: rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

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

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

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

rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
CONFIG_RCU_LAZY=y, the following scenarios will trigger WARN_ON_ONCE()
in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() functions:

        CPU2                                               CPU11
kthread
rcu_nocb_cb_kthread                                       ksys_write
rcu_do_batch                                              vfs_write
rcu_torture_timer_cb                                      proc_sys_write
__kmem_cache_free                                         proc_sys_call_handler
kmemleak_free                                             drop_caches_sysctl_handler
delete_object_full                                        drop_slab
__delete_object                                           shrink_slab
put_object                                                lazy_rcu_shrink_scan
call_rcu                                                  rcu_nocb_flush_bypass
__call_rcu_commn                                            rcu_nocb_bypass_lock
                                                            raw_spin_trylock(&rdp->nocb_bypass_lock) fail
                                                            atomic_inc(&rdp->nocb_lock_contended);
rcu_nocb_wait_contended                                     WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
 WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended))                                          |
                            |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|

Reproduce this bug with "echo 3 > /proc/sys/vm/drop_caches".

This commit therefore uses rcu_nocb_try_flush_bypass() instead of
rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan().  If the nocb_bypass
queue is being flushed, then rcu_nocb_try_flush_bypass will return
directly.

The Linux kernel CVE team has assigned CVE-2024-35929 to this issue.

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

	Fixed in 6.6.27 with commit 4d58c9fb45c7
	Fixed in 6.8.6 with commit 927d1f4f77e4
	Fixed in 6.9 with commit dda98810b552

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-2024-35929
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/4d58c9fb45c70e62c19e8be3f3605889c47601bc
	https://git.kernel.org/stable/c/927d1f4f77e4784ab3944a9df86ab14d1cd3185a
	https://git.kernel.org/stable/c/dda98810b552fc6bf650f4270edeebdc2f28bd3f