Re: [Qemu-block] Disconecting /dev/nbdX leaves stale partitions and device

[Stefan Hajnoczi]

On Thu, Jul 05, 2018 at 02:10:58AM +0300, Nir Soffer wrote:

CCing Eric and Paolo, who maintain QEMU's NBD code.

> I'm trying to access a qcow2 image via /dev/nbdX device.

Untrusted disk images should not be attached to the host using qemu-nbd
(or loopback devices) for security reasons.  If you hit this bug during
development or ad-hoc qemu-nbd usage, then that's fine.  But I just
wanted to post a reminder that production use cases should not use this
feature when dealing with untrusted disk images.

> I'm connecting a fedora 27 image (created using virt-builder) on a block
> device:
> 
> qemu-nbd -c /dev/nbd0 -f qcow2 -n --detect-zeroes=on --aio=native
> /dev/27837a03-64f9-4f2b-abb0-daa2195b01ae/acb196da-31fc-454d-856b-c31ab24715b3
> 
> It works, and I can read and write to the image via /dev/nbd0.
> 
> Disconnecting the device "works":
> 
> # qemu-nbd -d /dev/nbd0
> /dev/nbd0 disconnected
> 
> But when trying to connect the image again, I get this error:
> 
> # qemu-nbd -c /dev/nbd1 -f qcow2 -n --detect-zeroes=on --aio=native
> /dev/27837a03-64f9-4f2b-abb0-daa2195b01ae/acb196da-31fc-454d-856b-c31ab24715b3
> Failed to set NBD socket
> Disconnect client, due to: Unexpected end-of-file before all bytes were read
> 
> Turns out each time nbd devices is disconnected, it leaves the device
> and all the partitions on it "connected":
> 
> # lsblk
> ...
> nbd0
>            43:0    0    6G  0 disk
> ├─nbd0p1
>            43:1    0    1M  0 part
> ├─nbd0p2
>            43:2    0    1G  0 part
> ├─nbd0p3
>            43:3    0  615M  0 part
> └─nbd0p4
>            43:4    0  4.4G  0 part
> nbd1
>            43:32   0    6G  0 disk
> ├─nbd1p1
>            43:33   0    1M  0 part
> ├─nbd1p2
>            43:34   0    1G  0 part
> ├─nbd1p3
>            43:35   0  615M  0 part
> └─nbd1p4
>            43:36   0  4.4G  0 part
> 
> I tried also to do:
> 
> kpartx -d /dev/nbd1
> qemu-nbd -d /dev/nbd1
> 
> But I get the same results.
> 
> Testing on Fedora 28:
> kernel-4.17.2-200.fc28.x86_64
> qemu-img-2.12.0-0.5.rc1.fc28.x86_64
> 
> Nir

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