[PATCH v2 2/2] qemu: Move pid file of virtiofsd to stateDir

[Peng Liang <liangpeng10@xxxxxxxxxx>]

Libvirt will put the pid file of virtiofsd to per-domain directory.
However, the ownership of the per-domain directory is the user to run
the QEMU process and the user has the write permission of the directory.
If VM escape occurs, the attacker can
1. write arbitrary content to the pid file (if running QEMU using root),
   then the attacker can kill any process by writing appropriate pid to
   the pid file;
2. spoof the pid file (if running QEMU using a regular user), then the
   virtiofsd process will never be cleared even if the VM is destroyed.

So, move the pid file of virtiofsd from per-domain directory to
stateDir.

Signed-off-by: Peng Liang <liangpeng10@xxxxxxxxxx>
---
 src/qemu/qemu_virtiofs.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/qemu/qemu_virtiofs.c b/src/qemu/qemu_virtiofs.c
index 3ca45457c16e..0c12c5ea22a6 100644
--- a/src/qemu/qemu_virtiofs.c
+++ b/src/qemu/qemu_virtiofs.c
@@ -39,9 +39,9 @@
 VIR_LOG_INIT("qemu.virtiofs");
 
 
-char *
-qemuVirtioFSCreatePidFilename(virDomainObj *vm,
-                              const char *alias)
+static char *
+qemuVirtioFSCreatePidFilenameOld(virDomainObj *vm,
+                                 const char *alias)
 {
     qemuDomainObjPrivate *priv = vm->privateData;
     g_autofree char *name = NULL;
@@ -52,6 +52,19 @@ qemuVirtioFSCreatePidFilename(virDomainObj *vm,
 }
 
 
+char *
+qemuVirtioFSCreatePidFilename(virDomainObj *vm,
+                              const char *alias)
+{
+    qemuDomainObjPrivate *priv = vm->privateData;
+    g_autofree char *domname = virDomainDefGetShortName(vm->def);
+    g_autofree char *name = g_strdup_printf("%s-%s-fs", domname, alias);
+    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
+
+    return virPidFileBuildPath(cfg->stateDir, name);
+}
+
+
 char *
 qemuVirtioFSCreateSocketFilename(virDomainObj *vm,
                                  const char *alias)
@@ -283,6 +296,12 @@ qemuVirtioFSStop(virQEMUDriver *driver G_GNUC_UNUSED,
     if (!(pidfile = qemuVirtioFSCreatePidFilename(vm, fs->info.alias)))
         goto cleanup;
 
+    if (!virFileExists(pidfile)) {
+        g_free(pidfile);
+        if (!(pidfile = qemuVirtioFSCreatePidFilenameOld(vm, fs->info.alias)))
+            goto cleanup;
+    }
+
     if (virPidFileForceCleanupPathFull(pidfile, true) < 0) {
         VIR_WARN("Unable to kill virtiofsd process");
     } else {
-- 
2.31.1