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Date: Wed, 26 Aug 2015 13:10:50 +0800
From: Jason Wang <jasowang@redhat.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
CC: gleb@kernel.org, pbonzini@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, cornelia.huck@de.ibm.com
Subject: Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two
 buses
References: <1440493548-2971-1-git-send-email-jasowang@redhat.com> <1440493548-2971-2-git-send-email-jasowang@redhat.com> <20150825144925-mutt-send-email-mst@redhat.com>
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On 08/25/2015 07:51 PM, Michael S. Tsirkin wrote:
> On Tue, Aug 25, 2015 at 05:05:47PM +0800, Jason Wang wrote:
>> > We register wildcard mmio eventfd on two buses, one for KVM_MMIO_BUS
>> > and another is KVM_FAST_MMIO_BUS. This leads to issue:
>> > 
>> > - kvm_io_bus_destroy() knows nothing about the devices on two buses
>> >   points to a single dev. Which will lead double free [1] during exit.
>> > - wildcard eventfd ignores data len, so it was registered as a
>> >   kvm_io_range with zero length. This will fail the binary search in
>> >   kvm_io_bus_get_first_dev() when we try to emulate through
>> >   KVM_MMIO_BUS. This will cause userspace io emulation request instead
>> >   of a eventfd notification (virtqueue kick will be trapped by qemu
>> >   instead of vhost in this case).
>> > 
>> > Fixing this by don't register wildcard mmio eventfd on two
>> > buses. Instead, only register it in KVM_FAST_MMIO_BUS. This fixes the
>> > double free issue of kvm_io_bus_destroy(). For the arch/setups that
>> > does not utilize KVM_FAST_MMIO_BUS, before searching KVM_MMIO_BUS, try
>> > KVM_FAST_MMIO_BUS first to see it it has a match.
>> > 
>> > [1] Panic caused by double free:
>> > 
>> > CPU: 1 PID: 2894 Comm: qemu-system-x86 Not tainted 3.19.0-26-generic #28-Ubuntu
>> > Hardware name: LENOVO 2356BG6/2356BG6, BIOS G7ET96WW (2.56 ) 09/12/2013
>> > task: ffff88009ae0c4b0 ti: ffff88020e7f0000 task.ti: ffff88020e7f0000
>> > RIP: 0010:[<ffffffffc07e25d8>]  [<ffffffffc07e25d8>] ioeventfd_release+0x28/0x60 [kvm]
>> > RSP: 0018:ffff88020e7f3bc8  EFLAGS: 00010292
>> > RAX: dead000000200200 RBX: ffff8801ec19c900 RCX: 000000018200016d
>> > RDX: ffff8801ec19cf80 RSI: ffffea0008bf1d40 RDI: ffff8801ec19c900
>> > RBP: ffff88020e7f3bd8 R08: 000000002fc75a01 R09: 000000018200016d
>> > R10: ffffffffc07df6ae R11: ffff88022fc75a98 R12: ffff88021e7cc000
>> > R13: ffff88021e7cca48 R14: ffff88021e7cca50 R15: ffff8801ec19c880
>> > FS:  00007fc1ee3e6700(0000) GS:ffff88023e240000(0000) knlGS:0000000000000000
>> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> > CR2: 00007f8f389d8000 CR3: 000000023dc13000 CR4: 00000000001427e0
>> > Stack:
>> > ffff88021e7cc000 0000000000000000 ffff88020e7f3be8 ffffffffc07e2622
>> > ffff88020e7f3c38 ffffffffc07df69a ffff880232524160 ffff88020e792d80
>> >  0000000000000000 ffff880219b78c00 0000000000000008 ffff8802321686a8
>> > Call Trace:
>> > [<ffffffffc07e2622>] ioeventfd_destructor+0x12/0x20 [kvm]
>> > [<ffffffffc07df69a>] kvm_put_kvm+0xca/0x210 [kvm]
>> > [<ffffffffc07df818>] kvm_vcpu_release+0x18/0x20 [kvm]
>> > [<ffffffff811f69f7>] __fput+0xe7/0x250
>> > [<ffffffff811f6bae>] ____fput+0xe/0x10
>> > [<ffffffff81093f04>] task_work_run+0xd4/0xf0
>> > [<ffffffff81079358>] do_exit+0x368/0xa50
>> > [<ffffffff81082c8f>] ? recalc_sigpending+0x1f/0x60
>> > [<ffffffff81079ad5>] do_group_exit+0x45/0xb0
>> > [<ffffffff81085c71>] get_signal+0x291/0x750
>> > [<ffffffff810144d8>] do_signal+0x28/0xab0
>> > [<ffffffff810f3a3b>] ? do_futex+0xdb/0x5d0
>> > [<ffffffff810b7028>] ? __wake_up_locked_key+0x18/0x20
>> > [<ffffffff810f3fa6>] ? SyS_futex+0x76/0x170
>> > [<ffffffff81014fc9>] do_notify_resume+0x69/0xb0
>> > [<ffffffff817cb9af>] int_signal+0x12/0x17
>> > Code: 5d c3 90 0f 1f 44 00 00 55 48 89 e5 53 48 89 fb 48 83 ec 08 48 8b 7f 20 e8 06 d6 a5 c0 48 8b 43 08 48 8b 13 48 89 df 48 89 42 08 <48> 89 10 48 b8 00 01 10 00 00
>> > RIP  [<ffffffffc07e25d8>] ioeventfd_release+0x28/0x60 [kvm]
>> > RSP <ffff88020e7f3bc8>
>> > 
>> > Cc: Gleb Natapov <gleb@kernel.org>
>> > Cc: Paolo Bonzini <pbonzini@redhat.com>
>> > Cc: Michael S. Tsirkin <mst@redhat.com>
>> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>> > ---
>> > Changes from V2:
>> > - Tweak styles and comment suggested by Cornelia.
>> > Changes from v1:
>> > - change ioeventfd_bus_from_flags() to return KVM_FAST_MMIO_BUS when
>> >   needed to save lots of unnecessary changes.
>> > ---
>> >  virt/kvm/eventfd.c  | 31 +++++++++----------------------
>> >  virt/kvm/kvm_main.c | 16 ++++++++++++++--
>> >  2 files changed, 23 insertions(+), 24 deletions(-)
>> > 
>> > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> > index 9ff4193..c3ffdc3 100644
>> > --- a/virt/kvm/eventfd.c
>> > +++ b/virt/kvm/eventfd.c
>> > @@ -762,13 +762,16 @@ ioeventfd_check_collision(struct kvm *kvm, struct _ioeventfd *p)
>> >  	return false;
>> >  }
>> >  
>> > -static enum kvm_bus ioeventfd_bus_from_flags(__u32 flags)
>> > +static enum kvm_bus ioeventfd_bus_from_args(struct kvm_ioeventfd *args)
>> >  {
>> > -	if (flags & KVM_IOEVENTFD_FLAG_PIO)
>> > +	if (args->flags & KVM_IOEVENTFD_FLAG_PIO)
>> >  		return KVM_PIO_BUS;
>> > -	if (flags & KVM_IOEVENTFD_FLAG_VIRTIO_CCW_NOTIFY)
>> > +	if (args->flags & KVM_IOEVENTFD_FLAG_VIRTIO_CCW_NOTIFY)
>> >  		return KVM_VIRTIO_CCW_NOTIFY_BUS;
>> > -	return KVM_MMIO_BUS;
>> > +	/* When length is ignored, MMIO is put on a separate bus, for
>> > +	 * faster lookups.
>> > +	 */
>> > +	return args->len ? KVM_MMIO_BUS : KVM_FAST_MMIO_BUS;
>> >  }
>> >  
>> >  static int
>> > @@ -779,7 +782,7 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
>> >  	struct eventfd_ctx       *eventfd;
>> >  	int                       ret;
>> >  
>> > -	bus_idx = ioeventfd_bus_from_flags(args->flags);
>> > +	bus_idx = ioeventfd_bus_from_args(args);
>> >  	/* must be natural-word sized, or 0 to ignore length */
>> >  	switch (args->len) {
>> >  	case 0:
>> > @@ -843,16 +846,6 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
>> >  	if (ret < 0)
>> >  		goto unlock_fail;
>> >  
>> > -	/* When length is ignored, MMIO is also put on a separate bus, for
>> > -	 * faster lookups.
>> > -	 */
>> > -	if (!args->len && !(args->flags & KVM_IOEVENTFD_FLAG_PIO)) {
>> > -		ret = kvm_io_bus_register_dev(kvm, KVM_FAST_MMIO_BUS,
>> > -					      p->addr, 0, &p->dev);
>> > -		if (ret < 0)
>> > -			goto register_fail;
>> > -	}
>> > -
>> >  	kvm->buses[bus_idx]->ioeventfd_count++;
>> >  	list_add_tail(&p->list, &kvm->ioeventfds);
>> >  
>> > @@ -860,8 +853,6 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
>> >  
>> >  	return 0;
>> >  
>> > -register_fail:
>> > -	kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev);
>> >  unlock_fail:
>> >  	mutex_unlock(&kvm->slots_lock);
>> >  
>> > @@ -880,7 +871,7 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
>> >  	struct eventfd_ctx       *eventfd;
>> >  	int                       ret = -ENOENT;
>> >  
>> > -	bus_idx = ioeventfd_bus_from_flags(args->flags);
>> > +	bus_idx = ioeventfd_bus_from_args(args);
>> >  	eventfd = eventfd_ctx_fdget(args->fd);
>> >  	if (IS_ERR(eventfd))
>> >  		return PTR_ERR(eventfd);
>> > @@ -901,10 +892,6 @@ kvm_deassign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
>> >  			continue;
>> >  
>> >  		kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev);
>> > -		if (!p->length) {
>> > -			kvm_io_bus_unregister_dev(kvm, KVM_FAST_MMIO_BUS,
>> > -						  &p->dev);
>> > -		}
>> >  		kvm->buses[bus_idx]->ioeventfd_count--;
>> >  		ioeventfd_release(p);
>> >  		ret = 0;
>> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> > index 0d79fe8..3e3b3bc 100644
>> > --- a/virt/kvm/kvm_main.c
>> > +++ b/virt/kvm/kvm_main.c
>> > @@ -3152,8 +3152,8 @@ static int __kvm_io_bus_write(struct kvm_vcpu *vcpu, struct kvm_io_bus *bus,
>> >  }
>> >  
>> >  /* kvm_io_bus_write - called under kvm->slots_lock */
>> > -int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
>> > -		     int len, const void *val)
>> > +static int kvm_io_bus_write_idx(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx,
>> > +				gpa_t addr, int len, const void *val)
>> >  {
>> >  	struct kvm_io_bus *bus;
>> >  	struct kvm_io_range range;
>> > @@ -3169,6 +3169,18 @@ int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
>> >  	return r < 0 ? r : 0;
>> >  }
>> >  
>> > +int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
>> > +		     int len, const void *val)
>> > +{
>> > +	int r = 0;
>> > +
>> > +	if (bus_idx != KVM_MMIO_BUS ||
>> > +	    kvm_io_bus_write_idx(vcpu, KVM_FAST_MMIO_BUS, addr, 0, NULL) < 0)
>> > +		r = kvm_io_bus_write_idx(vcpu, bus_idx, addr, len, val);
>> > +
>> > +	return r < 0 ? r : 0;
>> > +}
>> > +
> Thinking more about this, invoking the 0-length write after
> the != 0 length one would be better: it would mean we only
> handle the userspace MMIO like this.

Right.

>
> All this calls for performance tests using kvm unit tests.
>

Ok, so if no obvious performance changes, do you agree this approach or
you still want to for two event fds?