[libvirt] [PATCH v2 1/2] m4: Run QEMU under a distro-specific user when possible

[Andrea Bolognani]

Our current defaults are root:wheel on FreeBSD and macOS, root:root
everywhere else.

Looking at what downstream distributions actually do, we can see that
these defaults are overriden the vast majority of the time, with a
number of variations showing up in the wild:

  * qemu:qemu -> Used by CentOS, Fedora, Gentoo, OpenSUSE, RHEL
                 and... As it turns out, our very own spec file :)

  * libvirt-qemu:libvirt-qemu -> Used by Debian.

  * libvirt-qemu:kvm -> Used by Ubuntu.

  * nobody:nobody -> Used by Arch Linux.

Based on this information, we can do a better job at integrating with
downstream packages: if the distro-specific user and group already
exist on the system then we use them, and if not (or we're building
on an unknown OS) we just use root:root as we would have before.

This change makes it less likely that people building from source
will end up running their guests as root, which from the security
point of view is a very desiderable outcome.

Signed-off-by: Andrea Bolognani <abologna at redhat.com>
---

Proof that I'm not making any of this up:

  * Alpine Linux
    https://github.com/alpinelinux/aports/blob/master/main/libvirt/APKBUILD

  * Arch Linux
    https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=libvirt-git#n113

  * CentOS
    https://git.centos.org/blob/rpms!libvirt.git/8d86411e5109d791edf49c7f43c08a06b80896af/SPECS!libvirt.spec#L226

  * Debian
    https://salsa.debian.org/libvirt-team/libvirt/blob/debian/sid/debian/rules#L94-95

  * Fedora
    https://src.fedoraproject.org/rpms/libvirt/blob/f29/f/libvirt.spec#_204

  * FreeBSD
    https://github.com/freebsd/freebsd-ports/blob/master/devel/libvirt/Makefile

  * Gentoo
    https://github.com/gentoo/gentoo/blob/master/app-emulation/libvirt/libvirt-5.1.0.ebuild#L296-L297

  * macOS (Homebrew)
    https://github.com/Homebrew/homebrew-core/blob/master/Formula/libvirt.rb

  * OpenSUSE
    https://build.opensuse.org/package/view_file/openSUSE:Leap:15.0:Update/libvirt/libvirt.spec?expand=1

  * Ubuntu
    https://git.launchpad.net/ubuntu/+source/libvirt/tree/debian/rules?h=ubuntu/disco#n99

  * Upstream :)
    https://libvirt.org/git/?p=libvirt.git;a=blob;f=libvirt.spec.in;h=b7a35a0fb14f3360eb795c4ec9b0e46171d2e4ec;hb=HEAD#l196

 m4/virt-driver-qemu.m4 | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/m4/virt-driver-qemu.m4 b/m4/virt-driver-qemu.m4
index cb05c34265..5d4ac66a4b 100644
--- a/m4/virt-driver-qemu.m4
+++ b/m4/virt-driver-qemu.m4
@@ -44,8 +44,34 @@ AC_DEFUN([LIBVIRT_DRIVER_CHECK_QEMU], [
     default_qemu_user=root
     default_qemu_group=wheel
   else
-    default_qemu_user=root
-    default_qemu_group=root
+    # Try to integrate gracefully with downstream packages by running QEMU
+    # processes under the same user and group they would
+    case $(grep ^ID= /etc/os-release 2>/dev/null) in
+      *arch*)
+        default_qemu_user=nobody
+        default_qemu_group=nobody
+        ;;
+      *centos*|*fedora*|*gentoo*|*rhel*|*suse*)
+        default_qemu_user=qemu
+        default_qemu_group=qemu
+        ;;
+      *debian*)
+        default_qemu_user=libvirt-qemu
+        default_qemu_group=libvirt-qemu
+        ;;
+      *ubuntu*)
+        default_qemu_user=libvirt-qemu
+        default_qemu_group=kvm
+        ;;
+    esac
+    # If the expected user and group don't exist, or we haven't hit any
+    # of the cases above because we're running on an unknown OS, the only
+    # sensible fallback is root:root
+    if ! getent passwd "$default_qemu_user" >/dev/null 2>&1 || \
+       ! getent group "$default_qemu_group" >/dev/null 2>&1; then
+      default_qemu_user=root
+      default_qemu_group=root
+    fi
   fi
 
   if test "x$with_qemu_user" = "xplatform dependent" ; then
-- 
2.20.1