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Re: [libvirt] [PATCH v2 3/4] secret: Properly handle @def after virSecretObjAdd in driver




On 07/25/2017 08:21 AM, Pavel Hrdina wrote:
> On Tue, Jul 25, 2017 at 01:36:20PM +0200, Pavel Hrdina wrote:
>> On Fri, Jul 14, 2017 at 10:04:41AM -0400, John Ferlan wrote:
>>> Since the virSecretObjListAdd technically consumes @def on success,
>>> the secretDefineXML should set @def = NULL immediately and process
>>> the remaining calls using a new @objdef variable. We can use use
>>> VIR_STEAL_PTR since we know the Add function just stores @def in
>>> obj->def.
>>>
>>> This fixes a possible double free of @def if the code jumps to
>>> restore_backup: and calls virSecretObjListRemove without setting
>>> def = NULL. In this case, the subsequent call to DefFree would
>>> succeed and free @def; however, the call to EndAPI would also
>>> call DefFree because the Unref done would be the last one for
>>> the @obj meaning the obj->def would be used to call DefFree,
>>> but it's already been free'd because @def wasn't managed right
>>> within this error path.
>>>
>>> Signed-off-by: John Ferlan <jferlan redhat com>
>>> ---
>>>  src/secret/secret_driver.c | 19 ++++++++++---------
>>>  1 file changed, 10 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/src/secret/secret_driver.c b/src/secret/secret_driver.c
>>> index 30124b4..77351d8 100644
>>> --- a/src/secret/secret_driver.c
>>> +++ b/src/secret/secret_driver.c
>>> @@ -210,6 +210,7 @@ secretDefineXML(virConnectPtr conn,
>>>  {
>>>      virSecretPtr ret = NULL;
>>>      virSecretObjPtr obj = NULL;
>>> +    virSecretDefPtr objdef;
>>
>> s/objdef/objDef/
>>
>> Reviewed-by: Pavel Hrdina <phrdina redhat com>
> 
> Actually this patch introduces a memory leak in case we restore the
> @backup definition.
> 
> virSecretObjListAdd() adds the @def into @obj and sets the old def
> into @backup, so far good.
> 
> VIR_STEAL_PTR(objdef, def) leaves the @def == NULL, still good
> 
> But if we in any point jump to "restore_backup" after the VIR_STEAL_PTR
> and the @backup != NULL we simply changes the @obj->def back to @backup
> and the new def is only in @objdef which is not freed anywhere.

I went through this in the previous review, but for the opposite
condition where backup == NULL (repeated for ease)

Without the stealing of objdef, when return from the Add function we
have "refcnt=2" and "lock=true".

For some reason we jump to restore_backup and call ObjListRemove which
returns and the object refcnt=1 and lock=false.

We fall into cleanup:

Call virSecretDefFree(def) where @def != NULL, so it free's it
Call virSecretObjEndAPI(&obj) which calls Unref, setting R=0 and calling
virSecretDefFree(obj->def)... But wait, we already did that because we
allowed the DefFree(def) to free something it didn't own.


> 
> Possible fix is to set @def = NULL after the virSecretObjListRemove()
> in restore_backup:
> 
>     ...
>     } else {
>         virSecretObjListRemove(driver->secrets, obj);
>         virObjectUnref(obj);
>         obj = NULL;
>         def = NULL;
>     }
>     ...
> 

The other fix to the leak you point out is:

    if (backup) {
        virSecretObjSetDef(obj, backup);
        def = objdef;
    } else {
        virSecretObjListRemove(driver->secrets, obj);
    }

> and there is no need to have yet another @objdef.

Unless you want the other condition of having two free's of @def

> 
> Pavel
> 

John


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