Libvirt will put the pid file of pr-helper 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
   pr-helper process will never be cleared even if the VM is destroyed.

So, move the pid file of pr-helper from per-domain directory to
stateDir just like the pid file of the domain.

Signed-off-by: Peng Liang <liangpeng10@huawei.com>
---
 src/qemu/qemu_process.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 1d0165af6daa..583f3ec76c7b 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -2859,9 +2859,11 @@ static char *
 qemuProcessBuildPRHelperPidfilePath(virDomainObj *vm)
 {
     qemuDomainObjPrivate *priv = vm->privateData;
-    const char *prdAlias = qemuDomainGetManagedPRAlias();
+    g_autofree char *domname = virDomainDefGetShortName(vm->def);
+    g_autofree char *prdName = g_strdup_printf("%s-%s", domname, qemuDomainGetManagedPRAlias());
+    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
 
-    return virPidFileBuildPath(priv->libDir, prdAlias);
+    return virPidFileBuildPath(cfg->stateDir, prdName);
 }
 
 
-- 
2.31.1

On 10/11/21 2:11 PM, Peng Liang wrote:
> Libvirt will put the pid file of pr-helper 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
>    pr-helper process will never be cleared even if the VM is destroyed.
> 
> So, move the pid file of pr-helper from per-domain directory to
> stateDir just like the pid file of the domain.
> 
> Signed-off-by: Peng Liang <liangpeng10@huawei.com>
> ---
>  src/qemu/qemu_process.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index 1d0165af6daa..583f3ec76c7b 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -2859,9 +2859,11 @@ static char *
>  qemuProcessBuildPRHelperPidfilePath(virDomainObj *vm)
>  {
>      qemuDomainObjPrivate *priv = vm->privateData;
> -    const char *prdAlias = qemuDomainGetManagedPRAlias();
> +    g_autofree char *domname = virDomainDefGetShortName(vm->def);
> +    g_autofree char *prdName = g_strdup_printf("%s-%s", domname, qemuDomainGetManagedPRAlias());
> +    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
>  
> -    return virPidFileBuildPath(priv->libDir, prdAlias);
> +    return virPidFileBuildPath(cfg->stateDir, prdName);
>  }

I don't think it is this simple. The pidfile path is not stored in
status XML but generated when starting/stopping PR helper process.
Therefore, if there is PR helper that was started with older libvirt its
pidfile is /var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid  but
after your change a different path is generated
/var/run/libvirt/qemu/1-fedora-pr-helper0.pid  and thus
qemuProcessKillManagedPRDaemon() does not kill the PR helper process and
leaves it behind.

One solution would be to start recording the pidfile path in status XML
and then when no path is found during parse we know that the old path
was used.

Michal

On 10/14/2021 6:10 PM, Michal Prívozník wrote:
> On 10/11/21 2:11 PM, Peng Liang wrote:
>> Libvirt will put the pid file of pr-helper 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
>>    pr-helper process will never be cleared even if the VM is destroyed.
>>
>> So, move the pid file of pr-helper from per-domain directory to
>> stateDir just like the pid file of the domain.
>>
>> Signed-off-by: Peng Liang <liangpeng10@huawei.com>
>> ---
>>  src/qemu/qemu_process.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>> index 1d0165af6daa..583f3ec76c7b 100644
>> --- a/src/qemu/qemu_process.c
>> +++ b/src/qemu/qemu_process.c
>> @@ -2859,9 +2859,11 @@ static char *
>>  qemuProcessBuildPRHelperPidfilePath(virDomainObj *vm)
>>  {
>>      qemuDomainObjPrivate *priv = vm->privateData;
>> -    const char *prdAlias = qemuDomainGetManagedPRAlias();
>> +    g_autofree char *domname = virDomainDefGetShortName(vm->def);
>> +    g_autofree char *prdName = g_strdup_printf("%s-%s", domname, qemuDomainGetManagedPRAlias());
>> +    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
>>  
>> -    return virPidFileBuildPath(priv->libDir, prdAlias);
>> +    return virPidFileBuildPath(cfg->stateDir, prdName);
>>  }
> 
> I don't think it is this simple. The pidfile path is not stored in
> status XML but generated when starting/stopping PR helper process.
> Therefore, if there is PR helper that was started with older libvirt its
> pidfile is /var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid  but
> after your change a different path is generated
> /var/run/libvirt/qemu/1-fedora-pr-helper0.pid  and thus
> qemuProcessKillManagedPRDaemon() does not kill the PR helper process and
> leaves it behind.
> 
> One solution would be to start recording the pidfile path in status XML
> and then when no path is found during parse we know that the old path
> was used.

Can we just change to use old path if the path returned by
qemuProcessBuildPRHelperPidfilePath does not exist?  For example:
diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index f95ed80fac43..0ee1a97f5571 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -2877,6 +2877,14 @@ qemuProcessKillManagedPRDaemon(virDomainObj *vm)
         return;
     }

+    if (!virFileExists(pidfile)) {
+        g_free(pidfile);
+        if (!(pidfile = qemuProcessBuildPRHelperPidfilePathOld(vm))) {
+            VIR_WARN("Unable to construct pr-helper pidfile path");
+            return;
+        }
+    }
+
     virErrorPreserveLast(&orig_err);
     if (virPidFileForceCleanupPath(pidfile) < 0) {
         VIR_WARN("Unable to kill pr-helper process");

qemuProcessBuildPRHelperPidfilePathOld will build the old path (e.g.,
/var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid).

Thanks,
Peng

> 
> Michal
> 
> .
> 

On 10/14/21 12:51 PM, Peng Liang wrote:
> On 10/14/2021 6:10 PM, Michal Prívozník wrote:
>> On 10/11/21 2:11 PM, Peng Liang wrote:
>>> Libvirt will put the pid file of pr-helper 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
>>>    pr-helper process will never be cleared even if the VM is destroyed.
>>>
>>> So, move the pid file of pr-helper from per-domain directory to
>>> stateDir just like the pid file of the domain.
>>>
>>> Signed-off-by: Peng Liang <liangpeng10@huawei.com>
>>> ---
>>>  src/qemu/qemu_process.c | 6 ++++--
>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>>> index 1d0165af6daa..583f3ec76c7b 100644
>>> --- a/src/qemu/qemu_process.c
>>> +++ b/src/qemu/qemu_process.c
>>> @@ -2859,9 +2859,11 @@ static char *
>>>  qemuProcessBuildPRHelperPidfilePath(virDomainObj *vm)
>>>  {
>>>      qemuDomainObjPrivate *priv = vm->privateData;
>>> -    const char *prdAlias = qemuDomainGetManagedPRAlias();
>>> +    g_autofree char *domname = virDomainDefGetShortName(vm->def);
>>> +    g_autofree char *prdName = g_strdup_printf("%s-%s", domname, qemuDomainGetManagedPRAlias());
>>> +    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
>>>  
>>> -    return virPidFileBuildPath(priv->libDir, prdAlias);
>>> +    return virPidFileBuildPath(cfg->stateDir, prdName);
>>>  }
>>
>> I don't think it is this simple. The pidfile path is not stored in
>> status XML but generated when starting/stopping PR helper process.
>> Therefore, if there is PR helper that was started with older libvirt its
>> pidfile is /var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid  but
>> after your change a different path is generated
>> /var/run/libvirt/qemu/1-fedora-pr-helper0.pid  and thus
>> qemuProcessKillManagedPRDaemon() does not kill the PR helper process and
>> leaves it behind.
>>
>> One solution would be to start recording the pidfile path in status XML
>> and then when no path is found during parse we know that the old path
>> was used.
> 
> Can we just change to use old path if the path returned by
> qemuProcessBuildPRHelperPidfilePath does not exist?  For example:
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index f95ed80fac43..0ee1a97f5571 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -2877,6 +2877,14 @@ qemuProcessKillManagedPRDaemon(virDomainObj *vm)
>          return;
>      }
> 
> +    if (!virFileExists(pidfile)) {
> +        g_free(pidfile);
> +        if (!(pidfile = qemuProcessBuildPRHelperPidfilePathOld(vm))) {
> +            VIR_WARN("Unable to construct pr-helper pidfile path");
> +            return;
> +        }
> +    }
> +
>      virErrorPreserveLast(&orig_err);
>      if (virPidFileForceCleanupPath(pidfile) < 0) {
>          VIR_WARN("Unable to kill pr-helper process");
> 
> qemuProcessBuildPRHelperPidfilePathOld will build the old path (e.g.,
> /var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid).

Yes, I think this may help.

Michal