On Mon, Mar 27, 2017 at 02:36:35PM +0200, Dmitry Vyukov wrote:
> On Tue, Mar 14, 2017 at 5:21 PM, Paul E. McKenney
> <paul...@linux.vnet.ibm.com> wrote:
> > On Tue, Mar 14, 2017 at 12:47:02AM -0700, Lance Roy wrote:
> >> I am not sure how the rcu_scheduler_active changes in __synchronize_srcu
> >> work,
> >> but there seem to be a few problems in them. First,
> >> "if (done && likely(!driving))" on line 453 doesn't appear to ever happen,
> >> as driving doesn't get set to false when srcu_reschedule is called. This
> >> seems
> >> like it could cause a race condition if another thread notices that
> >> ->running is
> >> false, adds itself to the queue, set ->running to true, and starts on its
> >> own
> >> grace period before the first thread acquires the lock again on line 469.
> >> Then
> >> the first thread will then acquire the lock, set running to false, and
> >> release
> >> the lock, resulting in an invalid state where ->running is false but the
> >> second
> >> thread is still trying to finish its grace period.
> >>
> >> Second, the while loop on line 455 seems to violate to rule that ->running
> >> shouldn't be false when there are entries in the queue. If a second thread
> >> adds
> >> itself to the queue while the first thread is driving SRCU inside that
> >> loop, and
> >> then the first thread finishes its own grace period and quits the loop, it
> >> will
> >> set ->running to false even though there is still a thread on the queue.
> >>
> >> The second issue requires rcu_scheduler_active to be RCU_SCHEDULER_INIT to
> >> occur, and as I don't what the assumptions during RCU_SCHEDULER_INIT are I
> >> don't
> >> know if it is actually a problem, but the first issue looks like it could
> >> occur
> >> at any time.
> >
> > Thank you for looking into this!
> >
> > I determined that my patch-order strategy was flawed, as it required
> > me to rewrite the mid-boot functionality several times. I therefore
> > removed the mid-boot commits. I will add them in later, but they will
> > use a rather different approach based on a grace-period sequence number
> > similar to that used by the expedited grace periods.
> >
> > Which should also teach me to be less aggressive about pushing new code
> > to -next. For a few weeks, anyway. ;-)
> >
> > Thanx, Paul
> >
> >> Thanks,
> >> Lance
> >>
> >> On Fri, 10 Mar 2017 14:26:09 -0800
> >> "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> >> > On Fri, Mar 10, 2017 at 08:29:55PM +0100, Andrey Konovalov wrote:
> >> > > On Fri, Mar 10, 2017 at 8:28 PM, Andrey Konovalov
> >> > > <andreyk...@google.com>
> >> > > wrote:
> >> > > > Kernel panic - not syncing: Fatal exception
> >> >
> >> > So the theory is that if !sp->running, all of SRCU's queues must be
> >> > empty.
> >> > So if you are holding ->queue_lock (with irqs disabled) and you see
> >> > !sp->running, and then you enqueue a callback on ->batch_check0, then
> >> > that callback must be the first in the list. And the code preceding
> >> > the WARN_ON() you triggered does in fact check and enqueue shile holding
> >> > ->queue_lock with irqs disabled.
> >> >
> >> > And rcu_batch_queue() does operate FIFO as required. (Otherwise,
> >> > srcu_barrier() would not work.)
> >> >
> >> > There are only three calls to rcu_batch_queue(), and the one involved
> >> > with
> >> > the WARN_ON() enqueues to ->batch_check0. The other two enqueue to
> >> > ->batch_queue. Callbacks move from ->batch_queue to ->batch_check0 to
> >> > ->batch_check1 to ->batch_done, so nothing should slip in front.
> >> >
> >> > Of course, if ->running were ever set to false with any of
> >> > ->batch_check0,
> >> > ->batch_check1, or ->batch_done non-empty, this WARN_ON() would trigger.
> >> > But srcu_reschedule() sets it to false only if all four batches are
> >> > empty (and checks and sets under ->queue_lock()), and all other cases
> >> > where it is set to false happen at initialization time, and also clear
> >> > out the queues. Of course, if someone raced an init_srcu_struct() with
> >> > either a call_srcu() or synchronize_srcu(), all bets are off. Now,
> >> > mmu_notifier.c does invoke init_srcu_struct() manually, but it does
> >> > so at subsys_initcall() time. Which -might- be after other things are
> >> > happening, so one "hail Mary" attempted fix is to remove
> >> > mmu_notifier_init()
> >> > and replace the "static struct srcu_struct srcu" with:
> >>
> >> >
> >> > DEFINE_STATIC_SRCU(srcu);
> >> >
> >> > But this might require changing the name -- I vaguely recall some
> >> > strangeness where the names of statically defined per-CPU variables need
> >> > to be globally unique even when static. Easy enough to do, though.
> >> > Might need a similar change to the "srcu" instances defined in vmd.c
> >> > and kvm_host.h -- assuming that this change helps.
> >> >
> >> > Another possibility is that something in SRCU is messing with either the
> >> > queues or the ->running field without holding ->queue_lock. And that
> >> > does
> >> > seem to be happening -- srcu_advance_batches() invokes rcu_batch_move()
> >> > without holding anything. Which seems like it could cause trouble
> >> > if someone else was invoking synchronize_srcu() concurrently. Those
> >> > particular invocations might be safe due to access only by a single
> >> > kthread/workqueue, given that all updates to ->batch_queue are protected
> >> > by ->queue_lock (aside from initialization).
> >> >
> >> > But ->batch_check0 is updated by __synchronize_srcu(), though protected
> >> > by ->queue_lock, and only if ->running is false, and with both the
> >> > check and the update protected by the same ->queue_lock critical section.
> >> > If ->running is false, then the workqueue cannot be running, so it
> >> > remains
> >> > to see if all other updates to ->batch_check0 are either with
> >> > ->queue_lock
> >> > held and ->running false on the one hand or from the workqueue handler
> >> > on the other:
> >> >
> >> > srcu_collect_new() updates with ->queue_lock held, but does not check
> >> > ->running. It is invoked only from process_srcu(), which in
> >> > turn is invoked only as a workqueue handler. The work is queued
> >> > from:
> >> >
> >> > call_srcu(), which does so with ->queue_lock held having just
> >> > set ->running to true.
> >> >
> >> > srcu_reschedule(), which invokes it if there are non-empty
> >> > queues. This is invoked from __synchronize_srcu()
> >> > in the case where it has set ->running to true
> >> > after finding the queues empty, which should imply
> >> > no other instances.
> >> >
> >> > It is also invoked from process_srcu(), which is
> >> > invoked only as a workqueue handler. (Yay
> >> > recursive inquiry!)
> >> >
> >> > srcu_advance_batches() updates without locks held. It is invoked as
> >> > follows:
> >> >
> >> > __synchronize_srcu() in the case where ->running was set, which
> >> > as noted before excludes workqueue handlers.
> >> >
> >> > process_srcu() which as noted before is only invoked from
> >> > a workqueue handler.
> >> >
> >> > So an SRCU workqueue is invoked only from a workqueue handler, or from
> >> > some other task that transitioned ->running from false to true while
> >> > holding ->queuelock. There should therefore only be one SRCU workqueue
> >> > per srcu_struct, so this should be safe. Though I hope that it can
> >> > be simplified a bit. :-/
> >> >
> >> > So the only suggestion I have at the moment is static definition of
> >> > the "srcu" variable. Lai, Josh, Steve, Mathieu, anything I missed?
> >> >
> >> > Thanx, Paul
>
>
>
> This happened on linux-next/65b2dc38291f9f27e5ec3b804d6eb3b5f79a3ce4
> and may be related.
> The report says that srcu subsystem still uses the srcu object after
> it has been freed. It can be a kvm fault as well.
Hmmm... I am not seeing a call to cleanup_srcu_struct() for the
->track_srcu field of the kvm_page_track_notifier_head structure.
Or is this structure immortal, so that it is never cleaned up?
Or am I just blind this morning?
In any case, freeing the kvm_page_track_notifier_head structure
without first invoking cleanup_srcu_struct() on its ->track_srcu
srcu_struct field could easily result in a use-after-free bug.
Thanx, Paul
> ==================================================================
> BUG: KASAN: use-after-free in debug_spin_unlock
> kernel/locking/spinlock_debug.c:97 [inline]
> BUG: KASAN: use-after-free in do_raw_spin_unlock+0x2ea/0x320
> kernel/locking/spinlock_debug.c:134
> Read of size 4 at addr ffff88014158a564 by task kworker/1:1/5712
>
> CPU: 1 PID: 5712 Comm: kworker/1:1 Not tainted 4.11.0-rc3-next-20170324+ #1
> Hardware name: Google Google Compute Engine/Google Compute Engine,
> BIOS Google 01/01/2011
> Workqueue: events_power_efficient process_srcu
> Call Trace:
> __dump_stack lib/dump_stack.c:16 [inline]
> dump_stack+0x2fb/0x40f lib/dump_stack.c:52
> print_address_description+0x7f/0x260 mm/kasan/report.c:250
> kasan_report_error mm/kasan/report.c:349 [inline]
> kasan_report.part.3+0x21f/0x310 mm/kasan/report.c:372
> kasan_report mm/kasan/report.c:392 [inline]
> __asan_report_load4_noabort+0x29/0x30 mm/kasan/report.c:392
> debug_spin_unlock kernel/locking/spinlock_debug.c:97 [inline]
> do_raw_spin_unlock+0x2ea/0x320 kernel/locking/spinlock_debug.c:134
> __raw_spin_unlock_irq include/linux/spinlock_api_smp.h:167 [inline]
> _raw_spin_unlock_irq+0x22/0x70 kernel/locking/spinlock.c:199
> spin_unlock_irq include/linux/spinlock.h:349 [inline]
> srcu_reschedule+0x1a1/0x260 kernel/rcu/srcu.c:582
> process_srcu+0x63c/0x11c0 kernel/rcu/srcu.c:600
> process_one_work+0xac0/0x1b00 kernel/workqueue.c:2097
> worker_thread+0x1b4/0x1300 kernel/workqueue.c:2231
> kthread+0x36c/0x440 kernel/kthread.c:231
> ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430
>
> Allocated by task 20961:
> save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
> save_stack+0x43/0xd0 mm/kasan/kasan.c:515
> set_track mm/kasan/kasan.c:527 [inline]
> kasan_kmalloc+0xaa/0xd0 mm/kasan/kasan.c:619
> kmem_cache_alloc_trace+0x10b/0x670 mm/slab.c:3635
> kmalloc include/linux/slab.h:492 [inline]
> kzalloc include/linux/slab.h:665 [inline]
> kvm_arch_alloc_vm include/linux/kvm_host.h:773 [inline]
> kvm_create_vm arch/x86/kvm/../../../virt/kvm/kvm_main.c:610 [inline]
> kvm_dev_ioctl_create_vm arch/x86/kvm/../../../virt/kvm/kvm_main.c:3161
> [inline]
> kvm_dev_ioctl+0x1bf/0x1460 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3205
> vfs_ioctl fs/ioctl.c:45 [inline]
> do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
> SYSC_ioctl fs/ioctl.c:700 [inline]
> SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
> entry_SYSCALL_64_fastpath+0x1f/0xbe
>
> Freed by task 20960:
> save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
> save_stack+0x43/0xd0 mm/kasan/kasan.c:515
> set_track mm/kasan/kasan.c:527 [inline]
> kasan_slab_free+0x6e/0xc0 mm/kasan/kasan.c:592
> __cache_free mm/slab.c:3511 [inline]
> kfree+0xd3/0x250 mm/slab.c:3828
> kvm_arch_free_vm include/linux/kvm_host.h:778 [inline]
> kvm_destroy_vm arch/x86/kvm/../../../virt/kvm/kvm_main.c:732 [inline]
> kvm_put_kvm+0x709/0x9a0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:747
> kvm_vm_release+0x42/0x50 arch/x86/kvm/../../../virt/kvm/kvm_main.c:758
> __fput+0x332/0x800 fs/file_table.c:209
> ____fput+0x15/0x20 fs/file_table.c:245
> task_work_run+0x197/0x260 kernel/task_work.c:116
> exit_task_work include/linux/task_work.h:21 [inline]
> do_exit+0x1a53/0x27c0 kernel/exit.c:878
> do_group_exit+0x149/0x420 kernel/exit.c:982
> get_signal+0x7d8/0x1820 kernel/signal.c:2318
> do_signal+0xd2/0x2190 arch/x86/kernel/signal.c:808
> exit_to_usermode_loop+0x21c/0x2d0 arch/x86/entry/common.c:157
> prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
> syscall_return_slowpath+0x4d3/0x570 arch/x86/entry/common.c:263
> entry_SYSCALL_64_fastpath+0xbc/0xbe
>
> The buggy address belongs to the object at ffff880141581640
> which belongs to the cache kmalloc-65536 of size 65536
> The buggy address is located 36644 bytes inside of
> 65536-byte region [ffff880141581640, ffff880141591640)
> The buggy address belongs to the page:
> page:ffffea000464b400 count:1 mapcount:0 mapping:ffff880141581640
> index:0x0 compound_mapcount: 0
> flags: 0x200000000008100(slab|head)
> raw: 0200000000008100 ffff880141581640 0000000000000000 0000000100000001
> raw: ffffea00064b1f20 ffffea000640fa20 ffff8801db800d00
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
> ffff88014158a400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88014158a480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88014158a500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ^
> ffff88014158a580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88014158a600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
>
