With qemu it is possible to run a guest from an unprivileged user but if
we wanted to communicate with the outside world we had to switch
to root.
We address this problem by introducing a new network backend and a new
network option for -net tap. This is less flexible when compared to
existing -net tap options because it relies on a helper with elevated
privileges to do the heavy lifting of allocating and attaching a tap
device to a bridge. We use a special purpose helper because we don't
want to elevate the privileges of more generic tools like brctl.
Qemu can be run with the default network helper as follows (in these cases
attaching the tap device to the default br0 bridge):
qemu linux.img -net bridge -net nic,model=virtio
qemu linux.img -net tap,helper=/usr/local/libexec/qemu-bridge-helper
-net nic,model=virtio
qemu linux.img -netdev bridge,id=hn0
-device virtio-net-pci,netdev=hn0,id=nic1
qemu linux.img -netdev
tap,helper=/usr/local/libexec/qemu-bridge-helper,id=hn0
-device virtio-net-pci,netdev=hn0,id=nic1
The default helper uses it's own ACL mechanism for access control, but
future network helpers could be developed, for example, to support PolicyKit
for access control.
More details are included in individual patches. The helper is broken into
a series of patches to improve reviewabilty.
v2:
- Updated signed-off-by's
- Updated author's email
- Set default bridge to br0
- Added -net bridge
- Updated ACL example
- Moved from libcap to libcap-ng
- Fail helper when libcap-ng not configured
v3:
- Use simple queue to store ACLs
- Added goto cleanup to helper's main
- Allow helper execution if libcap-ng not configured
- Completed static analysis and memory analysis on helper
v4:
- Update has_vnet_hdr() to return bool
- Update helper's main() to prevent errno clobbering
- Let Kernel cleanup helper's file descriptors
v5:
- Removed if statement with TUNGETIFF ioctl() from has_vnet_hdr()
- Added -netdev examples and udpated qemu -help netdev documentation
- Disallow vnet_hdr option with -net tap,helper
Corey Bryant (4):
Add basic version of bridge helper
Add access control support to qemu bridge helper
Add cap reduction support to enable use as SUID
Add support for net bridge
Makefile | 12 ++-
configure | 37 +++++
net.c | 29 ++++-
net.h | 3 +
net/tap.c | 191 +++++++++++++++++++++++-
net/tap.h | 3 +
qemu-bridge-helper.c | 402 ++++++++++++++++++++++++++++++++++++++++++++++++++
qemu-options.hx | 74 ++++++++--
8 files changed, 728 insertions(+), 23 deletions(-)
create mode 100644 qemu-bridge-helper.c