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The percpu refcount protects this structure, and we can have an atomic switch in progress when exiting. This makes it unsafe to just free the struct normally, and can trigger the following KASAN warning: BUG: KASAN: use-after-free in percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 Read of size 1 at addr ffff888181a19a30 by task swapper/0/0 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.0-rc4+ #5747 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014 Call Trace: <IRQ> dump_stack+0x76/0xa0 print_address_description.constprop.0+0x3b/0x60 ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 __kasan_report.cold+0x1a/0x3d ? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0 rcu_core+0x370/0x830 ? percpu_ref_exit+0x50/0x50 ? rcu_note_context_switch+0x7b0/0x7b0 ? run_rebalance_domains+0x11d/0x140 __do_softirq+0x10a/0x3e9 irq_exit+0xd5/0xe0 smp_apic_timer_interrupt+0x86/0x200 apic_timer_interrupt+0xf/0x20 </IRQ> RIP: 0010:default_idle+0x26/0x1f0 Fix this by punting the final exit and free of the struct to RCU, then we know that it's safe to do so. Jann suggested the approach of using a double rcu callback to achieve this. It's important that we do a nested call_rcu() callback, as otherwise the free could be ordered before the atomic switch, even if the latter was already queued. Reported-by: syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com Suggested-by: Jann Horn <jannh@google.com> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.