[libvirt] [PATCH v2] qemu: Fix crash in virDomainMemoryStats with old qemu

[John Ferlan]

On 02/05/2014 08:19 AM, Jiri Denemark wrote:
> If virDomainMemoryStats was run on a domain with virtio balloon driver
> running on an old qemu which supports QMP but does not support qom-list
> QMP command, libvirtd would crash. The reason is we did not check if
> qemuMonitorJSONGetObjectListPaths failed and moreover we even stored its
> result in an unsigned integer type.
> 
> Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
> ---
> 
> Notes:
>     version 2:
>     - use signed type for i and j to avoid comparison between signed and
>       unsigned types; gcc-- for not complaining about it
> 
>  src/qemu/qemu_monitor.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
> index a968901..a2769db 100644
> --- a/src/qemu/qemu_monitor.c
> +++ b/src/qemu/qemu_monitor.c
> @@ -1019,7 +1019,7 @@ qemuMonitorFindBalloonObjectPath(qemuMonitorPtr mon,
>                                   virDomainObjPtr vm,
>                                   const char *curpath)
>  {
> -    size_t i, j, npaths = 0, nprops = 0;
> +    ssize_t i, j, npaths = 0, nprops = 0;
>      int ret = 0;
>      char *nextpath = NULL;
>      qemuMonitorJSONListPathPtr *paths = NULL;
> @@ -1045,6 +1045,8 @@ qemuMonitorFindBalloonObjectPath(qemuMonitorPtr mon,
>      VIR_DEBUG("Searching for Balloon Object Path starting at %s", curpath);
>  
>      npaths = qemuMonitorJSONGetObjectListPaths(mon, curpath, &paths);
> +    if (npaths < 0)
> +        return -1;

Returning 0 from this function isn't necessarily bad - it means not
found still looking...  It's a recursive nightmare.

As for the non recursive callers...

For the qemuMonitorGetMemoryStats() path that's OK - it's allowed to
fallback to trying the "older" former method of calling "query-balloon"
in qemuMonitorJSONGetMemoryStats().  Returning -1 if "qom-list" isn't
found means we won't go the fallback route.

For the qemuMonitorSetMemoryStatsPeriod() returning 0 means don't even
try to set.

>  
>      for (i = 0; i < npaths && ret == 0; i++) {
>  
> @@ -1061,6 +1063,11 @@ qemuMonitorFindBalloonObjectPath(qemuMonitorPtr mon,
>               * then this version of qemu/kvm does not support the feature.
>               */
>              nprops = qemuMonitorJSONGetObjectListPaths(mon, nextpath, &bprops);
> +            if (nprops < 0) {
> +                ret = -1;
> +                goto cleanup;
> +            }
> +

Failure here wouldn't be because 'qom-list' doesn't exist, rather there
was some other property error or malformed return object.  Since not
finding "guest-stats-polling-interval" property for a
"link<virtio-balloon-pci>" object.

After the for loop that error is reported.  So if nprops <= 0, then we
fall through to that.  The other errors are still logged (right?), but
we report the error below.

>              for (j = 0; j < nprops; j++) {
>                  if (STREQ(bprops[j]->name, "guest-stats-polling-interval")) {
>                      VIR_DEBUG("Found Balloon Object Path %s", nextpath);
> 


FWIW: To Dan's comment - not sure how simple it would be to add a test
for this condition.

I'm still thinking the change in types is all that is necessary as there
is cause for this function to return 0 if "qom-list" doesn't exist.

John