blockjob: properly label disks for qemu block-commit

I finally have all the pieces in place to perform a block-commit with
SELinux enforcing.  There's still missing cleanup work when the commit
completes, but doing that requires tracking both the backing chain and
the base and top files within that chain in domain XML across libvirtd
restarts.  Furthermore, from a security standpoint, once you have
granted access, you must assume any damage that can be done will be
done; later revoking access is nice to minimize the window of damage,
but less important as it does not affect the fact that damage can be
done in the first place.  Therefore, deferring the revoke efforts until
we have better XML tracking of what chain operations are in effect,
including across a libvirtd restart, is reasonable.

* src/qemu/qemu_driver.c (qemuDomainBlockCommit): Label disks as
needed.
(qemuDomainPrepareDiskChainElement): Cast away const.

src/qemu/qemu_driver.c

@@ -10470,7 +10470,7 @@ qemuDomainPrepareDiskChainElement(struct qemud_driver *driver,
                                   virDomainObjPtr vm,
                                   virCgroupPtr cgroup,
                                   virDomainDiskDefPtr disk,
-                                  char *file,
+                                  const char *file,
                                   qemuDomainDiskChainMode mode)
 {
     /* The easiest way to label a single file with the same
@@ -10482,7 +10482,7 @@ qemuDomainPrepareDiskChainElement(struct qemud_driver *driver,
     bool origreadonly = disk->readonly;
     int ret = -1;
 
-    disk->src = file;
+    disk->src = (char *) file; /* casting away const is safe here */
     disk->format = VIR_STORAGE_FILE_RAW;
     disk->backingChain = NULL;
     disk->readonly = mode == VIR_DISK_CHAIN_READ_ONLY;
@@ -12700,6 +12700,7 @@ qemuDomainBlockCommit(virDomainPtr dom, const char *path, const char *base,
     virStorageFileMetadataPtr top_meta = NULL;
     const char *top_parent = NULL;
     const char *base_canon = NULL;
+    virCgroupPtr cgroup = NULL;
 
     virCheckFlags(VIR_DOMAIN_BLOCK_COMMIT_SHALLOW, -1);
 
@@ -12775,15 +12776,46 @@ qemuDomainBlockCommit(virDomainPtr dom, const char *path, const char *base,
         goto endjob;
     }
 
-    /* XXX We need to be changing the SELinux label on both 'base' and
-     * the parent of 'top', so that qemu can open(O_RDWR) those files
-     * for the duration of the commit.  */
+    /* For the commit to succeed, we must allow qemu to open both the
+     * 'base' image and the parent of 'top' as read/write; 'top' might
+     * not have a parent, or might already be read-write.  XXX It
+     * would also be nice to revert 'base' to read-only, as well as
+     * revoke access to files removed from the chain, when the commit
+     * operation succeeds, but doing that requires tracking the
+     * operation in XML across libvirtd restarts.  */
+    if (qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_DEVICES) &&
+        virCgroupForDomain(driver->cgroup, vm->def->name, &cgroup, 0) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Unable to find cgroup for %s"),
+                       vm->def->name);
+        goto endjob;
+    }
+    if (qemuDomainPrepareDiskChainElement(driver, vm, cgroup, disk, base_canon,
+                                          VIR_DISK_CHAIN_READ_WRITE) < 0 ||
+        (top_parent && top_parent != disk->src &&
+         qemuDomainPrepareDiskChainElement(driver, vm, cgroup, disk,
+                                           top_parent,
+                                           VIR_DISK_CHAIN_READ_WRITE) < 0))
+        goto endjob;
+
+    /* Start the commit operation.  */
     qemuDomainObjEnterMonitor(driver, vm);
     ret = qemuMonitorBlockCommit(priv->mon, device, top_canon, base_canon,
                                  bandwidth);
     qemuDomainObjExitMonitor(driver, vm);
 
 endjob:
+    if (ret < 0) {
+        /* Revert access to read-only, if possible.  */
+        qemuDomainPrepareDiskChainElement(driver, vm, cgroup, disk, base_canon,
+                                          VIR_DISK_CHAIN_READ_ONLY);
+        if (top_parent && top_parent != disk->src)
+            qemuDomainPrepareDiskChainElement(driver, vm, cgroup, disk,
+                                              top_parent,
+                                              VIR_DISK_CHAIN_READ_ONLY);
+    }
+    if (cgroup)
+        virCgroupFree(&cgroup);
     if (qemuDomainObjEndJob(driver, vm) == 0) {
         vm = NULL;
         goto cleanup;