[PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Yuan Yao]

In current packed virtqueue implementation, the avail_wrap_counter won't
flip, in the case when the driver supplies a descriptor chain with a
length equals to the queue size; total_sg == vq->packed.vring.num.

Let’s assume the following situation:
vq->packed.vring.num=4
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

Then the driver adds a descriptor chain containing 4 descriptors.

We expect the following result with avail_wrap_counter flipped:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 1

But, the current implementation gives the following result:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

To reproduce the bug, you can set a packed queue size as small as
possible, so that the driver is more likely to provide a descriptor
chain with a length equal to the packed queue size. For example, in
qemu run following commands:
sudo qemu-system-x86_64 \
-enable-kvm \
-nographic \
-kernel "path/to/kernel_image" \
-m 1G \
-drive file="path/to/rootfs",if=none,id=disk \
-device virtio-blk,drive=disk \
-drive file="path/to/disk_image",if=none,id=rwdisk \
-device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
indirect_desc=off \
-append "console=ttyS0 root=/dev/vda rw init=/bin/bash"

Inside the VM, create a directory and mount the rwdisk device on it. The
rwdisk will hang and mount operation will not complete.

This commit fixes the wrap counter error by flipping the
packed.avail_wrap_counter, when start of descriptor chain equals to the
end of descriptor chain (head == i).

Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
---

 drivers/virtio/virtio_ring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c5310eaf8b46..da1150d127c2 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
 		}
 	}
 
-	if (i < head)
+	if (i <= head)
 		vq->packed.avail_wrap_counter ^= 1;
 
 	/* We're using some buffers from the free list. */
-- 
2.41.0.640.ga95def55d0-goog

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Jason Wang]

On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
>
> In current packed virtqueue implementation, the avail_wrap_counter won't
> flip, in the case when the driver supplies a descriptor chain with a
> length equals to the queue size; total_sg == vq->packed.vring.num.
>
> Let’s assume the following situation:
> vq->packed.vring.num=4
> vq->packed.next_avail_idx: 1
> vq->packed.avail_wrap_counter: 0
>
> Then the driver adds a descriptor chain containing 4 descriptors.
>
> We expect the following result with avail_wrap_counter flipped:
> vq->packed.next_avail_idx: 1
> vq->packed.avail_wrap_counter: 1
>
> But, the current implementation gives the following result:
> vq->packed.next_avail_idx: 1
> vq->packed.avail_wrap_counter: 0
>
> To reproduce the bug, you can set a packed queue size as small as
> possible, so that the driver is more likely to provide a descriptor
> chain with a length equal to the packed queue size. For example, in
> qemu run following commands:
> sudo qemu-system-x86_64 \
> -enable-kvm \
> -nographic \
> -kernel "path/to/kernel_image" \
> -m 1G \
> -drive file="path/to/rootfs",if=none,id=disk \
> -device virtio-blk,drive=disk \
> -drive file="path/to/disk_image",if=none,id=rwdisk \
> -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
> indirect_desc=off \
> -append "console=ttyS0 root=/dev/vda rw init=/bin/bash"
>
> Inside the VM, create a directory and mount the rwdisk device on it. The
> rwdisk will hang and mount operation will not complete.
>
> This commit fixes the wrap counter error by flipping the
> packed.avail_wrap_counter, when start of descriptor chain equals to the
> end of descriptor chain (head == i).
>
> Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
> Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
> ---
>
>  drivers/virtio/virtio_ring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c5310eaf8b46..da1150d127c2 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>                 }
>         }
>
> -       if (i < head)
> +       if (i <= head)
>                 vq->packed.avail_wrap_counter ^= 1;

Would it be better to move the flipping to the place where we flip
avail_used_flags?

                        if ((unlikely(++i >= vq->packed.vring.num))) {
                                i = 0;
                                vq->packed.avail_used_flags ^=
                                        1 << VRING_PACKED_DESC_F_AVAIL |
                                        1 << VRING_PACKED_DESC_F_USED;
                        }

Thanks

>
>         /* We're using some buffers from the free list. */
> --
> 2.41.0.640.ga95def55d0-goog
>

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Michael S. Tsirkin]

On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote:
> On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
> >
> > In current packed virtqueue implementation, the avail_wrap_counter won't
> > flip, in the case when the driver supplies a descriptor chain with a
> > length equals to the queue size; total_sg == vq->packed.vring.num.
> >
> > Let’s assume the following situation:
> > vq->packed.vring.num=4
> > vq->packed.next_avail_idx: 1
> > vq->packed.avail_wrap_counter: 0
> >
> > Then the driver adds a descriptor chain containing 4 descriptors.
> >
> > We expect the following result with avail_wrap_counter flipped:
> > vq->packed.next_avail_idx: 1
> > vq->packed.avail_wrap_counter: 1
> >
> > But, the current implementation gives the following result:
> > vq->packed.next_avail_idx: 1
> > vq->packed.avail_wrap_counter: 0
> >
> > To reproduce the bug, you can set a packed queue size as small as
> > possible, so that the driver is more likely to provide a descriptor
> > chain with a length equal to the packed queue size. For example, in
> > qemu run following commands:
> > sudo qemu-system-x86_64 \
> > -enable-kvm \
> > -nographic \
> > -kernel "path/to/kernel_image" \
> > -m 1G \
> > -drive file="path/to/rootfs",if=none,id=disk \
> > -device virtio-blk,drive=disk \
> > -drive file="path/to/disk_image",if=none,id=rwdisk \
> > -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
> > indirect_desc=off \
> > -append "console=ttyS0 root=/dev/vda rw init=/bin/bash"
> >
> > Inside the VM, create a directory and mount the rwdisk device on it. The
> > rwdisk will hang and mount operation will not complete.
> >
> > This commit fixes the wrap counter error by flipping the
> > packed.avail_wrap_counter, when start of descriptor chain equals to the
> > end of descriptor chain (head == i).
> >
> > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
> > Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
> > ---
> >
> >  drivers/virtio/virtio_ring.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index c5310eaf8b46..da1150d127c2 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >                 }
> >         }
> >
> > -       if (i < head)
> > +       if (i <= head)
> >                 vq->packed.avail_wrap_counter ^= 1;
> 
> Would it be better to move the flipping to the place where we flip
> avail_used_flags?

I think I prefer this patch for stable, refactoring can
be done on top.

>                         if ((unlikely(++i >= vq->packed.vring.num))) {
>                                 i = 0;
>                                 vq->packed.avail_used_flags ^=
>                                         1 << VRING_PACKED_DESC_F_AVAIL |
>                                         1 << VRING_PACKED_DESC_F_USED;
>                         }
> 
> Thanks
> 
> >
> >         /* We're using some buffers from the free list. */
> > --
> > 2.41.0.640.ga95def55d0-goog
> >

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Jason Wang]

On Tue, Aug 8, 2023 at 1:59 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote:
> > On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
> > >
> > > In current packed virtqueue implementation, the avail_wrap_counter won't
> > > flip, in the case when the driver supplies a descriptor chain with a
> > > length equals to the queue size; total_sg == vq->packed.vring.num.
> > >
> > > Let’s assume the following situation:
> > > vq->packed.vring.num=4
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 0
> > >
> > > Then the driver adds a descriptor chain containing 4 descriptors.
> > >
> > > We expect the following result with avail_wrap_counter flipped:
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 1
> > >
> > > But, the current implementation gives the following result:
> > > vq->packed.next_avail_idx: 1
> > > vq->packed.avail_wrap_counter: 0
> > >
> > > To reproduce the bug, you can set a packed queue size as small as
> > > possible, so that the driver is more likely to provide a descriptor
> > > chain with a length equal to the packed queue size. For example, in
> > > qemu run following commands:
> > > sudo qemu-system-x86_64 \
> > > -enable-kvm \
> > > -nographic \
> > > -kernel "path/to/kernel_image" \
> > > -m 1G \
> > > -drive file="path/to/rootfs",if=none,id=disk \
> > > -device virtio-blk,drive=disk \
> > > -drive file="path/to/disk_image",if=none,id=rwdisk \
> > > -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
> > > indirect_desc=off \
> > > -append "console=ttyS0 root=/dev/vda rw init=/bin/bash"
> > >
> > > Inside the VM, create a directory and mount the rwdisk device on it. The
> > > rwdisk will hang and mount operation will not complete.
> > >
> > > This commit fixes the wrap counter error by flipping the
> > > packed.avail_wrap_counter, when start of descriptor chain equals to the
> > > end of descriptor chain (head == i).
> > >
> > > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
> > > Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
> > > ---
> > >
> > >  drivers/virtio/virtio_ring.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index c5310eaf8b46..da1150d127c2 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > >                 }
> > >         }
> > >
> > > -       if (i < head)
> > > +       if (i <= head)
> > >                 vq->packed.avail_wrap_counter ^= 1;
> >
> > Would it be better to move the flipping to the place where we flip
> > avail_used_flags?
>
> I think I prefer this patch for stable, refactoring can
> be done on top.

Ok. So

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

>
> >                         if ((unlikely(++i >= vq->packed.vring.num))) {
> >                                 i = 0;
> >                                 vq->packed.avail_used_flags ^=
> >                                         1 << VRING_PACKED_DESC_F_AVAIL |
> >                                         1 << VRING_PACKED_DESC_F_USED;
> >                         }
> >
> > Thanks
> >
> > >
> > >         /* We're using some buffers from the free list. */
> > > --
> > > 2.41.0.640.ga95def55d0-goog
> > >
>

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Jason Wang]

On Tue, Aug 8, 2023 at 1:59 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
>
> Since there is a check for 0-length chain in code;BUG_ON(total_sg == 0), we won’t get a 0-length chain in practice. So, I think use (i <= head) makes the commit as small as possible.

Ok, offered ack in other mail.

Thanks

>
>
> Best regards
>
>
>
> On Tue, Aug 8, 2023 at 2:43 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
>> >
>> > In current packed virtqueue implementation, the avail_wrap_counter won't
>> > flip, in the case when the driver supplies a descriptor chain with a
>> > length equals to the queue size; total_sg == vq->packed.vring.num.
>> >
>> > Let’s assume the following situation:
>> > vq->packed.vring.num=4
>> > vq->packed.next_avail_idx: 1
>> > vq->packed.avail_wrap_counter: 0
>> >
>> > Then the driver adds a descriptor chain containing 4 descriptors.
>> >
>> > We expect the following result with avail_wrap_counter flipped:
>> > vq->packed.next_avail_idx: 1
>> > vq->packed.avail_wrap_counter: 1
>> >
>> > But, the current implementation gives the following result:
>> > vq->packed.next_avail_idx: 1
>> > vq->packed.avail_wrap_counter: 0
>> >
>> > To reproduce the bug, you can set a packed queue size as small as
>> > possible, so that the driver is more likely to provide a descriptor
>> > chain with a length equal to the packed queue size. For example, in
>> > qemu run following commands:
>> > sudo qemu-system-x86_64 \
>> > -enable-kvm \
>> > -nographic \
>> > -kernel "path/to/kernel_image" \
>> > -m 1G \
>> > -drive file="path/to/rootfs",if=none,id=disk \
>> > -device virtio-blk,drive=disk \
>> > -drive file="path/to/disk_image",if=none,id=rwdisk \
>> > -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
>> > indirect_desc=off \
>> > -append "console=ttyS0 root=/dev/vda rw init=/bin/bash"
>> >
>> > Inside the VM, create a directory and mount the rwdisk device on it. The
>> > rwdisk will hang and mount operation will not complete.
>> >
>> > This commit fixes the wrap counter error by flipping the
>> > packed.avail_wrap_counter, when start of descriptor chain equals to the
>> > end of descriptor chain (head == i).
>> >
>> > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
>> > Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
>> > ---
>> >
>> >  drivers/virtio/virtio_ring.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> > index c5310eaf8b46..da1150d127c2 100644
>> > --- a/drivers/virtio/virtio_ring.c
>> > +++ b/drivers/virtio/virtio_ring.c
>> > @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>> >                 }
>> >         }
>> >
>> > -       if (i < head)
>> > +       if (i <= head)
>> >                 vq->packed.avail_wrap_counter ^= 1;
>>
>> Would it be better to move the flipping to the place where we flip
>> avail_used_flags?
>>
>>                         if ((unlikely(++i >= vq->packed.vring.num))) {
>>                                 i = 0;
>>                                 vq->packed.avail_used_flags ^=
>>                                         1 << VRING_PACKED_DESC_F_AVAIL |
>>                                         1 << VRING_PACKED_DESC_F_USED;
>>                         }
>>
>> Thanks
>>
>> >
>> >         /* We're using some buffers from the free list. */
>> > --
>> > 2.41.0.640.ga95def55d0-goog
>> >
>>

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Michael S. Tsirkin]

On Tue, Aug 08, 2023 at 05:37:29PM +0900, Yuan Yao wrote:
> Thank you for reviewing, sent a patch with your ack.

Don't do this pls. And don't top post please.

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Michael S. Tsirkin]

Nope - it will be in the next linux release.

On Mon, Aug 28, 2023 at 12:33:46PM +0900, Yuan Yao wrote:
> I'm writing to confirm the process for the patch, since I'm not sure if this
> thread is done for this patch.
> Do I need any further steps to take to make this patch be launched?
> 
> On Tue, Aug 8, 2023 at 7:00 PM Yuan Yao <yuanyaogoog@chromium.org> wrote:
> 
>     Sorry for the confusing mail, I didn't understand how it works.
>     Thanks
> 
>     On Tue, Aug 8, 2023 at 6:13 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>         On Tue, Aug 08, 2023 at 05:37:29PM +0900, Yuan Yao wrote:
>         > Thank you for reviewing, sent a patch with your ack.
> 
>         Don't do this pls. And don't top post please.
> 
> 

[PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Yuan Yao]

In current packed virtqueue implementation, the avail_wrap_counter won't
flip, in the case when the driver supplies a descriptor chain with a
length equals to the queue size; total_sg == vq->packed.vring.num.

Let’s assume the following situation:
vq->packed.vring.num=4
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

Then the driver adds a descriptor chain containing 4 descriptors.

We expect the following result with avail_wrap_counter flipped:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 1

But, the current implementation gives the following result:
vq->packed.next_avail_idx: 1
vq->packed.avail_wrap_counter: 0

To reproduce the bug, you can set a packed queue size as small as
possible, so that the driver is more likely to provide a descriptor
chain with a length equal to the packed queue size. For example, in
qemu run following commands:
sudo qemu-system-x86_64 \
-enable-kvm \
-nographic \
-kernel "path/to/kernel_image" \
-m 1G \
-drive file="path/to/rootfs",if=none,id=disk \
-device virtio-blk,drive=disk \
-drive file="path/to/disk_image",if=none,id=rwdisk \
-device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\
indirect_desc=off \
-append "console=ttyS0 root=/dev/vda rw init=/bin/bash"

Inside the VM, create a directory and mount the rwdisk device on it. The
rwdisk will hang and mount operation will not complete.

This commit fixes the wrap counter error by flipping the
packed.avail_wrap_counter, when start of descriptor chain equals to the
end of descriptor chain (head == i).

Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support")
Signed-off-by: Yuan Yao <yuanyaogoog@chromium.org>
Acked-by: Jason Wang <jasowang@redhat.com>
---

 drivers/virtio/virtio_ring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c5310eaf8b46..da1150d127c2 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
 		}
 	}
 
-	if (i < head)
+	if (i <= head)
 		vq->packed.avail_wrap_counter ^= 1;
 
 	/* We're using some buffers from the free list. */
-- 
2.41.0.640.ga95def55d0-goog

Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed

[Michael S. Tsirkin]

On Tue, Aug 08, 2023 at 07:05:38PM +0900, Yuan Yao wrote:
> Sorry, but please ignore this thread.

ok, dropped.