qemu_cgroup: Handle device mapper targets properly

https://bugzilla.redhat.com/show_bug.cgi?id=1557769

Problem with device mapper targets is that there can be several
other devices 'hidden' behind them. For instance, /dev/dm-1 can
consist of /dev/sda, /dev/sdb and /dev/sdc. Therefore, when
setting up devices CGroup and namespaces we have to take this
into account.

This bug was exposed after Linux kernel was fixed. Initially,
kernel used different functions for getting block device in
open() and ioctl(). While CGroup permissions were checked in the
former case, due to a bug in kernel they were not checked in the
latter case. This changed with the upstream commit of
519049afead4f7c3e6446028c41e99fde958cc04 (v4.16-rc5~11^2~4).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_cgroup.c

@@ -37,6 +37,7 @@
 #include "virtypedparam.h"
 #include "virnuma.h"
 #include "virsystemd.h"
+#include "virdevmapper.h"
 
 #define VIR_FROM_THIS VIR_FROM_QEMU
 
@@ -60,7 +61,10 @@ qemuSetupImagePathCgroup(virDomainObjPtr vm,
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
     int perms = VIR_CGROUP_DEVICE_READ;
-    int ret;
+    char **targetPaths = NULL;
+    size_t i;
+    int rv;
+    int ret = -1;
 
     if (!virCgroupHasController(priv->cgroup, VIR_CGROUP_CONTROLLER_DEVICES))
         return 0;
@@ -71,12 +75,41 @@ qemuSetupImagePathCgroup(virDomainObjPtr vm,
     VIR_DEBUG("Allow path %s, perms: %s",
               path, virCgroupGetDevicePermsString(perms));
 
-    ret = virCgroupAllowDevicePath(priv->cgroup, path, perms, true);
+    rv = virCgroupAllowDevicePath(priv->cgroup, path, perms, true);
 
     virDomainAuditCgroupPath(vm, priv->cgroup, "allow", path,
                              virCgroupGetDevicePermsString(perms),
-                             ret);
+                             rv);
+    if (rv < 0)
+        goto cleanup;
 
+    if (rv > 0) {
+        /* @path is neither character device nor block device. */
+        ret = 0;
+        goto cleanup;
+    }
+
+    if (virDevMapperGetTargets(path, &targetPaths) < 0 &&
+        errno != ENOSYS && errno != EBADF) {
+        virReportSystemError(errno,
+                             _("Unable to get devmapper targets for %s"),
+                             path);
+        goto cleanup;
+    }
+
+    for (i = 0; targetPaths && targetPaths[i]; i++) {
+        rv = virCgroupAllowDevicePath(priv->cgroup, targetPaths[i], perms, false);
+
+        virDomainAuditCgroupPath(vm, priv->cgroup, "allow", targetPaths[i],
+                                 virCgroupGetDevicePermsString(perms),
+                                 rv);
+        if (rv < 0)
+            goto cleanup;
+    }
+
+    ret = 0;
+ cleanup:
+    virStringListFree(targetPaths);
     return ret;
 }
 
@@ -131,6 +164,13 @@ qemuTeardownImageCgroup(virDomainObjPtr vm,
     virDomainAuditCgroupPath(vm, priv->cgroup, "deny", src->path,
                              virCgroupGetDevicePermsString(perms), ret);
 
+    /* If you're looking for a counter part to
+     * qemuSetupImagePathCgroup you're at the right place.
+     * However, we can't just blindly deny all the device mapper
+     * targets of src->path because they might still be used by
+     * another disk in domain. Just like we are not removing
+     * disks from namespace. */
+
     return ret;
 }