Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 31, 2015 at 11:12:07AM +0800, Jason Wang wrote:
> 
> 
> On 08/26/2015 01:10 PM, Jason Wang wrote:
> > 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.
> >
> 
> Using current unittest. This patch is about 2.9% slower than before, and
> invoking 0-length write after is still 1.1% slower (mmio-datamatch-eventfd).
> 
> /patch/result/-+%/
> /base/2957/0/
> /V3/3043/+2.9%/
> /V3+invoking != 0 length first/2990/+1.1%/
> 
> So looks like the best method is not searching KVM_FAST_MMIO_BUS during
> KVM_MMIO_BUS. Instead, let userspace to register both datamatch and
> wildcard in this case. Does this sound good to you?

No - we can't change userspace.

-- 
MST