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Date:   Mon, 16 Jan 2017 16:48:25 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Steve Rutherford <srutherford@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: kvm: use-after-free in process_srcu



----- Original Message -----
> From: "Dmitry Vyukov" <dvyukov@...gle.com>
> To: "Steve Rutherford" <srutherford@...gle.com>
> Cc: "syzkaller" <syzkaller@...glegroups.com>, "Paolo Bonzini" <pbonzini@...hat.com>, "Radim Krčmář"
> <rkrcmar@...hat.com>, "KVM list" <kvm@...r.kernel.org>, "LKML" <linux-kernel@...r.kernel.org>
> Sent: Monday, January 16, 2017 10:34:26 PM
> Subject: Re: kvm: use-after-free in process_srcu
> 
> On Sun, Jan 15, 2017 at 6:11 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > On Fri, Jan 13, 2017 at 10:19 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >> On Fri, Jan 13, 2017 at 4:30 AM, Steve Rutherford
> >> <srutherford@...gle.com> wrote:
> >>> I'm not that familiar with the kernel's workqueues, but this seems
> >>> like the classic "callback outlives the memory it references"
> >>> use-after-free, where the process_srcu callback is outliving struct
> >>> kvm (which contains the srcu_struct). If that's right, then calling
> >>> srcu_barrier (which should wait for all of the call_srcu callbacks to
> >>> complete, which are what enqueue the process_srcu callbacks) before
> >>> cleanup_srcu_struct in kvm_destroy_vm probably fixes this.
> >>>
> >>> The corresponding patch to virt/kvm/kvm_main.c looks something like:
> >>> static void kvm_destroy_vm(struct kvm *kvm)
> >>> {
> >>> ...
> >>>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> >>>                 kvm_free_memslots(kvm, kvm->memslots[i]);
> >>> +      srcu_barrier(&kvm->irq_srcu);
> >>>         cleanup_srcu_struct(&kvm->irq_srcu);
> >>> +      srcu_barrier(&kvm->srcu);
> >>>         cleanup_srcu_struct(&kvm->srcu);
> >>> ...
> >>>
> >>>
> >>> Since we don't have a repro, this obviously won't be readily testable.
> >>> I find srcu subtle enough that I don't trust my reasoning fully (in
> >>> particular, I don't trust that waiting for all of the call_srcu
> >>> callbacks to complete also waits for all of the process_srcu
> >>> callbacks). Someone else know if that's the case?
> >>
> >>
> >> From the function description it looks like it should do the trick:
> >>
> >> 514 /**
> >> 515  * srcu_barrier - Wait until all in-flight call_srcu() callbacks
> >> complete.
> >> 516  * @sp: srcu_struct on which to wait for in-flight callbacks.
> >> 517  */
> >> 518 void srcu_barrier(struct srcu_struct *sp)
> >>
> >> I see this failure happening several times per day. I've applied your
> >> patch locally and will check if I see these failures happening.
> >
> >
> > I have not seen the crash in 3 days, when usually I see several
> > crashes per night. So I think we can consider that the patch fixes the
> > crash:
> >
> > Tested-by: Dmitry Vyukov <dvyukov@...gle.com>
> 
> 
> Unfortunately I hit it again with the patch applied. It definitely
> happens less frequently now, but still happens:

Try this:

diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 9b9cdd549caa..ef5599c65299 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -283,6 +283,7 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 {
 	if (WARN_ON(srcu_readers_active(sp)))
 		return; /* Leakage unless caller handles error. */
+	flush_delayed_work(&sp->work);
 	free_percpu(sp->per_cpu_ref);
 	sp->per_cpu_ref = NULL;
 }

I think it should subsume Steve's patch, but I'm not 101% sure.  We
will have to run this through Paul.

Paolo

> 
> BUG: KASAN: use-after-free in rcu_batch_empty kernel/rcu/srcu.c:64
> [inline] at addr ffff88006ada0218
> BUG: KASAN: use-after-free in rcu_batch_dequeue kernel/rcu/srcu.c:75
> [inline] at addr ffff88006ada0218
> BUG: KASAN: use-after-free in srcu_invoke_callbacks
> kernel/rcu/srcu.c:624 [inline] at addr ffff88006ada0218
> BUG: KASAN: use-after-free in process_srcu+0x275/0x280
> kernel/rcu/srcu.c:672 at addr ffff88006ada0218
> Read of size 8 by task kworker/2:2/1073
> CPU: 2 PID: 1073 Comm: kworker/2:2 Not tainted 4.10.0-rc3+ #164
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Workqueue: events_power_efficient process_srcu
> Call Trace:
>  __dump_stack lib/dump_stack.c:15 [inline]
>  dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>  kasan_object_err+0x1c/0x70 mm/kasan/report.c:165
>  print_address_description mm/kasan/report.c:203 [inline]
>  kasan_report_error mm/kasan/report.c:287 [inline]
>  kasan_report+0x1b6/0x460 mm/kasan/report.c:307
>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:333
>  rcu_batch_empty kernel/rcu/srcu.c:64 [inline]
>  rcu_batch_dequeue kernel/rcu/srcu.c:75 [inline]
>  srcu_invoke_callbacks kernel/rcu/srcu.c:624 [inline]
>  process_srcu+0x275/0x280 kernel/rcu/srcu.c:672
>  process_one_work+0xbd0/0x1c10 kernel/workqueue.c:2098
>  worker_thread+0x223/0x1990 kernel/workqueue.c:2232
>  kthread+0x326/0x3f0 kernel/kthread.c:227
>  ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:427
> Object at ffff88006ada00c0, in cache kmalloc-16384 size: 16384
> Allocated:
> PID = 32313
> [<ffffffff812b2686>] save_stack_trace+0x16/0x20
> arch/x86/kernel/stacktrace.c:57
> [<ffffffff81a0e3d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:502
> [<ffffffff81a0e69a>] set_track mm/kasan/kasan.c:514 [inline]
> [<ffffffff81a0e69a>] kasan_kmalloc+0xaa/0xd0 mm/kasan/kasan.c:605
> [<ffffffff81a0aeeb>] kmem_cache_alloc_trace+0x10b/0x670 mm/slab.c:3629
> [<ffffffff8107910e>] kvm_arch_alloc_vm include/linux/slab.h:490 [inline]
> [<ffffffff8107910e>] kvm_create_vm
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:613 [inline]
> [<ffffffff8107910e>] kvm_dev_ioctl_create_vm
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:3175 [inline]
> [<ffffffff8107910e>] kvm_dev_ioctl+0x1be/0x11b0
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:3219
> [<ffffffff81aa55bf>] vfs_ioctl fs/ioctl.c:43 [inline]
> [<ffffffff81aa55bf>] do_vfs_ioctl+0x1bf/0x1790 fs/ioctl.c:683
> [<ffffffff81aa6c1f>] SYSC_ioctl fs/ioctl.c:698 [inline]
> [<ffffffff81aa6c1f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
> [<ffffffff841c9941>] entry_SYSCALL_64_fastpath+0x1f/0xc2
> Freed:
> PID = 32310
> [<ffffffff812b2686>] save_stack_trace+0x16/0x20
> arch/x86/kernel/stacktrace.c:57
> [<ffffffff81a0e3d3>] save_stack+0x43/0xd0 mm/kasan/kasan.c:502
> [<ffffffff81a0ed0f>] set_track mm/kasan/kasan.c:514 [inline]
> [<ffffffff81a0ed0f>] kasan_slab_free+0x6f/0xb0 mm/kasan/kasan.c:578
> [<ffffffff81a0cbc3>] __cache_free mm/slab.c:3505 [inline]
> [<ffffffff81a0cbc3>] kfree+0xd3/0x250 mm/slab.c:3822
> [<ffffffff81070579>] kvm_arch_free_vm include/linux/kvm_host.h:781 [inline]
> [<ffffffff81070579>] kvm_destroy_vm
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:744 [inline]
> [<ffffffff81070579>] kvm_put_kvm+0x719/0x9a0
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:759
> [<ffffffff81070952>] kvm_vm_release+0x42/0x50
> arch/x86/kvm/../../../virt/kvm/kvm_main.c:770
> [<ffffffff81a5f872>] __fput+0x332/0x7f0 fs/file_table.c:208
> [<ffffffff81a5fdb5>] ____fput+0x15/0x20 fs/file_table.c:244
> [<ffffffff814ca25a>] task_work_run+0x18a/0x260 kernel/task_work.c:116
> [<ffffffff81454a57>] exit_task_work include/linux/task_work.h:21 [inline]
> [<ffffffff81454a57>] do_exit+0x18e7/0x28a0 kernel/exit.c:839
> [<ffffffff8145a469>] do_group_exit+0x149/0x420 kernel/exit.c:943
> [<ffffffff81489730>] get_signal+0x7e0/0x1820 kernel/signal.c:2313
> [<ffffffff8127ca92>] do_signal+0xd2/0x2190 arch/x86/kernel/signal.c:807
> [<ffffffff81007900>] exit_to_usermode_loop+0x200/0x2a0
> arch/x86/entry/common.c:156
> [<ffffffff81009413>] prepare_exit_to_usermode
> arch/x86/entry/common.c:190 [inline]
> [<ffffffff81009413>] syscall_return_slowpath+0x4d3/0x570
> arch/x86/entry/common.c:259
> [<ffffffff841c99e2>] entry_SYSCALL_64_fastpath+0xc0/0xc2
> Memory state around the buggy address:
>  ffff88006ada0100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff88006ada0180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88006ada0200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                             ^
>  ffff88006ada0280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff88006ada0300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> Disabling lock debugging due to kernel taint
> Kernel panic - not syncing: panic_on_warn set ...
> 
> CPU: 2 PID: 1073 Comm: kworker/2:2 Tainted: G    B           4.10.0-rc3+ #164
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Workqueue: events_power_efficient process_srcu
> Call Trace:
>  __dump_stack lib/dump_stack.c:15 [inline]
>  dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>  panic+0x1fb/0x412 kernel/panic.c:179
>  kasan_end_report+0x5b/0x60 mm/kasan/report.c:144
>  kasan_report_error mm/kasan/report.c:291 [inline]
>  kasan_report+0x386/0x460 mm/kasan/report.c:307
>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:333
>  rcu_batch_empty kernel/rcu/srcu.c:64 [inline]
>  rcu_batch_dequeue kernel/rcu/srcu.c:75 [inline]
>  srcu_invoke_callbacks kernel/rcu/srcu.c:624 [inline]
>  process_srcu+0x275/0x280 kernel/rcu/srcu.c:672
>  process_one_work+0xbd0/0x1c10 kernel/workqueue.c:2098
>  worker_thread+0x223/0x1990 kernel/workqueue.c:2232
>  kthread+0x326/0x3f0 kernel/kthread.c:227
>  ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:427
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled
> 

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