qemu: keep capabilities when running QEMU as root

When QEMU uid/gid is set to non-root this is pointless as if we just
used a regular setuid/setgid call, the process will have all its
capabilities cleared anyway by the kernel.

When QEMU uid/gid is set to root, this is almost (always?) never
what people actually want. People make QEMU run as root in order
to access some privileged resource that libvirt doesn't support
yet and this often requires capabilities. As a result they have
to go find the qemu.conf param to turn this off. This is not
viable for libguestfs - they want to control everything via the
XML security label to request running as root regardless of the
qemu.conf settings for user/group.

Clearing capabilities was implemented originally because there
was a proposal in Fedora to change permissions such that root,
with no capabilities would not be able to compromise the system.
ie a locked down root account. This never went anywhere though,
and as a result clearing capabilities when running as root does
not really get us any security benefit AFAICT. The root user
can easily do something like create a cronjob, which will then
faithfully be run with full capabilities, trivially bypassing
the restriction we place.

IOW, our clearing of capabilities is both useless from a security
POV, and breaks valid use cases when people need to run as root.

This removes the clear_emulator_capabilities configuration
option from qemu.conf, and always runs QEMU with capabilities
when root.  The behaviour when non-root is unchanged.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/drvqemu.html.in

@@ -187,41 +187,29 @@ chmod o+x /path/to/directory
       </li>
     </ul>
 
+    <p>
+      The libvirt maintainers <strong>strongly recommend against</strong>
+      running QEMU as the root user/group. This should not be required
+      in most supported usage scenarios, as libvirt will generally do the
+      right thing to grant QEMU access to files it is permitted to
+      use when it is running non-root.
+    </p>
+
     <h3><a id="securitycap">Linux process capabilities</a></h3>
 
     <p>
-      The libvirt QEMU driver has a build time option allowing it to use
-      the <a href="http://people.redhat.com/sgrubb/libcap-ng/index.html">libcap-ng</a>
-      library to manage process capabilities. If this build option is
-      enabled, then the QEMU driver will use this to ensure that all
-      process capabilities are dropped before executing a QEMU virtual
-      machine. Process capabilities are what gives the 'root' account
-      its high power, in particular the CAP_DAC_OVERRIDE capability
-      is what allows a process running as 'root' to access files owned
-      by any user.
+      In versions of libvirt prior to 6.0.0, even if QEMU was configured
+      to run as the root user / group, libvirt would strip all process
+      capabilities. This meant that QEMU could only read/write files
+      owned by root, or with open permissions. In reality, stripping
+      capabilities did not have any security benefit, as it was trivial
+      to get commands to run in another context with full capabilities,
+      for example, by creating a cronjob.
     </p>
-
     <p>
-      If the QEMU driver is configured to run virtual machines as non-root,
-      then they will already lose all their process capabilities at time
-      of startup. The Linux capability feature is thus aimed primarily at
-      the scenario where the QEMU processes are running as root. In this
-      case, before launching a QEMU virtual machine, libvirtd will use
-      libcap-ng APIs to drop all process capabilities. It is important
-      for administrators to note that this implies the QEMU process will
-      <strong>only</strong> be able to access files owned by root, and
-      not files owned by any other user.
-    </p>
-
-    <p>
-      Thus, if a vendor / distributor has configured their libvirt package
-      to run as 'qemu' by default, a number of changes will be required
-      before an administrator can change a host to run guests as root.
-      In particular it will be necessary to change ownership on the
-      directories <code>/var/run/libvirt/qemu/</code>,
-      <code>/var/lib/libvirt/qemu/</code> and
-      <code>/var/cache/libvirt/qemu/</code> back to root, in addition
-      to changing the <code>/etc/libvirt/qemu.conf</code> settings.
+      Thus since 6.0.0, if QEMU is running as root, it will keep all
+      process capabilities. Behaviour when QEMU is running non-root
+      is unchanged, it still has no capabilities.
     </p>
 
     <h3><a id="securityselinux">SELinux basic confinement</a></h3>

src/qemu/libvirtd_qemu.aug

@@ -86,7 +86,6 @@ module Libvirtd_qemu =
                  | bool_entry "auto_start_bypass_cache"
 
    let process_entry = str_entry "hugetlbfs_mount"
-                 | bool_entry "clear_emulator_capabilities"
                  | str_entry "bridge_helper"
                  | str_entry "pr_helper"
                  | str_entry "slirp_helper"
@@ -129,6 +128,12 @@ module Libvirtd_qemu =
    let swtpm_entry = str_entry "swtpm_user"
                 | str_entry "swtpm_group"
 
+   (* Entries that used to exist in the config which are now
+    * deleted. We keep on parsing them so we don't break
+    * ability to parse old configs after upgrade
+    *)
+   let obsolete_entry = bool_entry "clear_emulator_capabilities"
+
    let capability_filters_entry = str_array_entry "capability_filters"
 
    (* Each entry in the config is one of the following ... *)
@@ -153,6 +158,7 @@ module Libvirtd_qemu =
              | nbd_entry
              | swtpm_entry
              | capability_filters_entry
+             | obsolete_entry
 
    let comment = [ label "#comment" . del /#[ \t]*/ "# " .  store /([^ \t\n][^\n]*)?/ . del /\n/ "\n" ]
    let empty = [ label "#empty" . eol ]

src/qemu/qemu.conf

@@ -583,17 +583,6 @@
 #bridge_helper = "/usr/libexec/qemu-bridge-helper"
 
 
-
-# If clear_emulator_capabilities is enabled, libvirt will drop all
-# privileged capabilities of the QEMU/KVM emulator. This is enabled by
-# default.
-#
-# Warning: Disabling this option means that a compromised guest can
-# exploit the privileges and possibly do damage to the host.
-#
-#clear_emulator_capabilities = 1
-
-
 # If enabled, libvirt will have QEMU set its process name to
 # "qemu:VM_NAME", where VM_NAME is the name of the VM. The QEMU
 # process will appear as "qemu:VM_NAME" in process listings and

src/qemu/qemu_conf.c

@@ -234,8 +234,6 @@ virQEMUDriverConfigPtr virQEMUDriverConfigNew(bool privileged)
     cfg->prHelperName = g_strdup(QEMU_PR_HELPER);
     cfg->slirpHelperName = g_strdup(QEMU_SLIRP_HELPER);
 
-    cfg->clearEmulatorCapabilities = true;
-
     cfg->securityDefaultConfined = true;
     cfg->securityRequireConfined = false;
 
@@ -602,8 +600,6 @@ virQEMUDriverConfigLoadProcessEntry(virQEMUDriverConfigPtr cfg,
         }
     }
 
-    if (virConfGetValueBool(conf, "clear_emulator_capabilities", &cfg->clearEmulatorCapabilities) < 0)
-        return -1;
     if (virConfGetValueString(conf, "bridge_helper", &cfg->bridgeHelperName) < 0)
         return -1;
 

src/qemu/qemu_conf.h

@@ -161,7 +161,6 @@ struct _virQEMUDriverConfig {
     bool relaxedACS;
     bool vncAllowHostAudio;
     bool nogfxAllowHostAudio;
-    bool clearEmulatorCapabilities;
     bool setProcessName;
 
     unsigned int maxProcesses;

src/qemu/qemu_domain.c

@@ -9512,8 +9512,7 @@ void qemuDomainObjCheckTaint(virQEMUDriverPtr driver,
     bool custom_hypervisor_feat = false;
 
     if (virQEMUDriverIsPrivileged(driver) &&
-        (!cfg->clearEmulatorCapabilities ||
-         cfg->user == 0 ||
+        (cfg->user == 0 ||
          cfg->group == 0))
         qemuDomainObjTaint(driver, obj, VIR_DOMAIN_TAINT_HIGH_PRIVILEGES, logCtxt);
 

src/qemu/qemu_process.c

@@ -6812,11 +6812,6 @@ qemuProcessLaunch(virConnectPtr conn,
     if (qemuDomainCreateNamespace(driver, vm) < 0)
         goto cleanup;
 
-    VIR_DEBUG("Clear emulator capabilities: %d",
-              cfg->clearEmulatorCapabilities);
-    if (cfg->clearEmulatorCapabilities)
-        virCommandClearCaps(cmd);
-
     VIR_DEBUG("Setting up raw IO");
     if (qemuProcessSetupRawIO(driver, vm, cmd) < 0)
         goto cleanup;

src/qemu/test_libvirtd_qemu.aug.in

@@ -72,7 +72,6 @@ module Test_libvirtd_qemu =
 { "auto_start_bypass_cache" = "0" }
 { "hugetlbfs_mount" = "/dev/hugepages" }
 { "bridge_helper" = "/usr/libexec/qemu-bridge-helper" }
-{ "clear_emulator_capabilities" = "1" }
 { "set_process_name" = "1" }
 { "max_processes" = "0" }
 { "max_files" = "0" }