USB-hotplugging fails with "failed to load cgroup BPF prog: Operation not permitted" on cgroups v2

Tue Jan 21 11:53:49 UTC 2020

On Mon, Jan 20, 2020 at 09:00:15PM +0100, Pol Van Aubel wrote:
> Hi,
> 
> Quoting Pavel Hrdina (2020-01-20 14:29:36)
> > On Sat, Jan 18, 2020 at 11:17:11PM +0100, Pol Van Aubel wrote:
> > > Hi all,
> > > 
> > > I've disabled cgroups v1 on my system with the kernel boot option
> > > "systemd.unified_cgroup_hierarchy=1". Since doing so, USB hotplugging
> > > fails to work, seemingly due to a permissions problem with BPF. Please
> > > note that the technique I'm going to describe worked just fine for
> > > hotplugging USB devices to running domains until this change.
> > > Attaching / detaching USB devices when the domain is down still works as
> > > expected.
> > > 
> > > I get the same error when attaching a device in virt-manager, as I do
> > > when running the following command:
> > > 
> > > sudo virsh attach-device wenger /dev/stdin --persistent <<END
> > > <hostdev mode='subsystem' type='usb' managed='yes'>
> > >   <source startupPolicy='optional'>
> > >     <vendor id='0x046d' />
> > >     <product id='0xc215' />
> > >   </source>
> > > </hostdev>
> > > END
> > > 
> > > This returns
> > > error: Failed to attach device from /dev/stdin
> > > error: failed to load cgroup BPF prog: Operation not permitted
> > > 
> > > 
> > > virt-manager returns basically the same error, but for completeness'
> > > sake, here it is:
> > > 
> > > failed to load cgroup BPF prog: Operation not permitted
> > > 
> > > Traceback (most recent call last):
> > >   File "/usr/share/virt-manager/virtManager/addhardware.py", line 1327, in _add_device
> > >     self.vm.attach_device(dev)
> > >   File "/usr/share/virt-manager/virtManager/object/domain.py", line 920, in attach_device
> > >     self._backend.attachDevice(devxml)
> > >   File "/usr/lib/python3.8/site-packages/libvirt.py", line 590, in attachDevice
> > >     if ret == -1: raise libvirtError ('virDomainAttachDevice() failed', dom=self)
> > > libvirt.libvirtError: failed to load cgroup BPF prog: Operation not permitted
> > > 
> > > 
> > > Now, libvirtd is running as root, so I don't understand why any
> > > operation on BPF programs is not permitted. I've dug into libvirt's code
> > > a bit to see what is throwing this error and it boils down to
> > > <https://github.com/libvirt/libvirt/blob/7d608469621a3fda72dff2a89308e68cc9fb4c9a/src/util/vircgroupv2devices.c#L292-L296>
> > > and
> > > <https://github.com/libvirt/libvirt/blob/02bf7cc68bfc76242f02d23e73cad36618f3f790/src/util/virbpf.c#L54>
> > > but I have no clue what that syscall is doing, so that's where my
> > > debugging capability basically ends.
> > > 
> > > Maybe this is something as simple as setting the right ACL somewhere. I
> > > haven't touched /etc/libvirt/qemu.conf except for setting nvram. There
> > > *is* something about cgroup_device_acl there but afaict that's for
> > > cgroups v1, when there was still a device cgroup controller. Any help
> > > would be greatly appreciated.
> > > 
> > > 
> > > Domain log files:
> > > Upon execution of the above commands, nothing gets added to the domain
> > > log in /var/log/qemu/wenger.log, so I've decided they're likely
> > > irrelevant to the issue. Please ask for any additional info required.
> > > 
> > > 
> > > System information:
> > > Arch Linux, (normal) kernel 5.4.11
> > > libvirt 5.10.0
> > > qemu 4.2.0, using KVM.
> > > Host system is x86_64 on an intel 5820k.
> > > Guest system is probably irrelevant, but is Windows 10 on the same.
> > > 
> > > 
> > > Possibly relevant kernel build options:
> > > $ zgrep BPF /proc/config.gz                                                       
> > > [22:55:52]: zgrep BPF /proc/config.gz
> > > 
> > > CONFIG_CGROUP_BPF=y
> > > CONFIG_BPF=y
> > > CONFIG_BPF_SYSCALL=y
> > > CONFIG_BPF_JIT_ALWAYS_ON=y
> > > CONFIG_IPV6_SEG6_BPF=y
> > > CONFIG_NETFILTER_XT_MATCH_BPF=m
> > > # CONFIG_BPFILTER is not set
> > > CONFIG_NET_CLS_BPF=m
> > > CONFIG_NET_ACT_BPF=m
> > > CONFIG_BPF_JIT=y
> > > CONFIG_BPF_STREAM_PARSER=y
> > > CONFIG_LWTUNNEL_BPF=y
> > > CONFIG_HAVE_EBPF_JIT=y
> > > CONFIG_BPF_EVENTS=y
> > > # CONFIG_BPF_KPROBE_OVERRIDE is not set
> > > # CONFIG_TEST_BPF is not set
> > 
> > Hi
> > 
> > I've installed clean archlinux to try this out and it works as expected,
> > I'm able to attach USB device into a VM.
> > 
> > My system env is mostly the same as yours except for kernel version:
> > 
> >     kernel 5.4.13
> >     libvirt 5.10.0
> >     qemu 4.2.0, using KVM.
> > 
> > Please enable libvirt debug logs [1] and share the output with us.
> 
> I've updated to 5.4.13 and created a barebones VM without storage to
> reproduce the behaviour. libvirtd debug logs are attached. There appear
> to be two BPF failures of the same BPF program (?). The first is on line
> 23209, which appears to be part of machine startup, and which I don't
> actually notice. The second one is where I manually add the USB device,
> on line 30599.
> 
> Thanks,

Thanks for the logs, but it did not help to figure out where the issue
is.  I was hoping to see some error output from the syscall but the line
that should contain it is empty:

2020-01-20 19:47:15.589+0000: 8579: debug : virBPFLoadProg:78 :

Can you please check system logs and output of dmesg?

I've managed to run into this article [1] that explains that even if you
have all permissions and no SELinux you can still be blocked by
something called kernel_lockdown and it should appear in dmesg.

Pavel

[1] <https://gehrcke.de/2019/09/running-an-ebpf-program-may-require-lifting-the-kernel-lockdown/>
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