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 <[email protected]>
Acked-by: Jason Wang <[email protected]>
---
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
On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <[email protected]> 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 <[email protected]>
> ---
>
> 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
>
On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote:
> On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <[email protected]> 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 <[email protected]>
> > ---
> >
> > 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
> >
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.
On Tue, Aug 8, 2023 at 1:59 PM Michael S. Tsirkin <[email protected]> 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 <[email protected]> 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 <[email protected]>
> > > ---
> > >
> > > 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 <[email protected]>
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
> > >
>
On Tue, Aug 8, 2023 at 1:59 PM Yuan Yao <[email protected]> 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 <[email protected]> wrote:
>>
>> On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao <[email protected]> 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 <[email protected]>
>> > ---
>> >
>> > 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
>> >
>>