[PATCH] vboxguest: check for private_data before trying to close it.

[PATCH] vboxguest: check for private_data before trying to close it.

[Young Xiao]

vbg_misc_device_close doesn't check that filp->private_data is non-NULL
before trying to close_session, where vbg_core_close_session uses pointer
session whithout checking, too. That can cause an oops in certain error
conditions that can occur on open or lookup before the private_data is set.

This vulnerability is similar to CVE-2011-1771.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
---
 drivers/virt/vboxguest/vboxguest_linux.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virt/vboxguest/vboxguest_linux.c b/drivers/virt/vboxguest/vboxguest_linux.c
index 6e8c0f1..b03c16f 100644
--- a/drivers/virt/vboxguest/vboxguest_linux.c
+++ b/drivers/virt/vboxguest/vboxguest_linux.c
@@ -88,8 +88,10 @@ static int vbg_misc_device_user_open(struct inode *inode, struct file *filp)
  */
 static int vbg_misc_device_close(struct inode *inode, struct file *filp)
 {
-	vbg_core_close_session(filp->private_data);
-	filp->private_data = NULL;
+	if (file->private_data != NULL) {
+		vbg_core_close_session(filp->private_data);
+		filp->private_data = NULL;
+	}
 	return 0;
 }
 
-- 
2.7.4

Re: [PATCH] vboxguest: check for private_data before trying to close it.

[Hans de Goede]

Hi,

On 28-05-19 14:47, Young Xiao wrote:
> vbg_misc_device_close doesn't check that filp->private_data is non-NULL
> before trying to close_session, where vbg_core_close_session uses pointer
> session whithout checking, too. That can cause an oops in certain error
> conditions that can occur on open or lookup before the private_data is set.
> 
> This vulnerability is similar to CVE-2011-1771.

How is this in anyway related to CVE-2011-1771 ????

That CVE is about a filesystems lookup method including a direct open
of the file for performance reasons and if that direct open fails, doing
a fput, which is how it ends up with a file with private_date being NULL,
see:

https://bugzilla.redhat.com/show_bug.cgi?id=703016

"the problem is that CIFS doesn't do O_DIRECT at all, so when you try to open a file with it you get back -EINVAL. CIFS can also do open on lookup in some cases. In that case, fput will be called on the filp, which has not yet had its private_data set."

The vboxguest code on the other hand is not a filessystem and as such
does not have a dentry lookup method at all!

It's close method is part of the file_operations struct for a character
device using the misc_device framework which guarantees that private_data
is always set.

Regards,

Hans





> 
> Signed-off-by: Young Xiao <92siuyang@gmail.com>
> ---
>   drivers/virt/vboxguest/vboxguest_linux.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virt/vboxguest/vboxguest_linux.c b/drivers/virt/vboxguest/vboxguest_linux.c
> index 6e8c0f1..b03c16f 100644
> --- a/drivers/virt/vboxguest/vboxguest_linux.c
> +++ b/drivers/virt/vboxguest/vboxguest_linux.c
> @@ -88,8 +88,10 @@ static int vbg_misc_device_user_open(struct inode *inode, struct file *filp)
>    */
>   static int vbg_misc_device_close(struct inode *inode, struct file *filp)
>   {
> -	vbg_core_close_session(filp->private_data);
> -	filp->private_data = NULL;
> +	if (file->private_data != NULL) {
> +		vbg_core_close_session(filp->private_data);
> +		filp->private_data = NULL;
> +	}
>   	return 0;
>   }
>   
> 

Re: [PATCH] vboxguest: check for private_data before trying to close it.

[Yang Xiao]

Based on your explanation, file->private_data can not be NULL before
call vbg_core_close_session method all the time, since
file->private_data is always set.
Am I right?

On Tue, May 28, 2019 at 9:19 PM Hans de Goede <hdegoede@redhat.com> wrote:
>
> Hi,
>
> On 28-05-19 14:47, Young Xiao wrote:
> > vbg_misc_device_close doesn't check that filp->private_data is non-NULL
> > before trying to close_session, where vbg_core_close_session uses pointer
> > session whithout checking, too. That can cause an oops in certain error
> > conditions that can occur on open or lookup before the private_data is set.
> >
> > This vulnerability is similar to CVE-2011-1771.
>
> How is this in anyway related to CVE-2011-1771 ????
>
> That CVE is about a filesystems lookup method including a direct open
> of the file for performance reasons and if that direct open fails, doing
> a fput, which is how it ends up with a file with private_date being NULL,
> see:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=703016
>
> "the problem is that CIFS doesn't do O_DIRECT at all, so when you try to open a file with it you get back -EINVAL. CIFS can also do open on lookup in some cases. In that case, fput will be called on the filp, which has not yet had its private_data set."
>
> The vboxguest code on the other hand is not a filessystem and as such
> does not have a dentry lookup method at all!
>
> It's close method is part of the file_operations struct for a character
> device using the misc_device framework which guarantees that private_data
> is always set.
>
> Regards,
>
> Hans
>
>
>
>
>
> >
> > Signed-off-by: Young Xiao <92siuyang@gmail.com>
> > ---
> >   drivers/virt/vboxguest/vboxguest_linux.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virt/vboxguest/vboxguest_linux.c b/drivers/virt/vboxguest/vboxguest_linux.c
> > index 6e8c0f1..b03c16f 100644
> > --- a/drivers/virt/vboxguest/vboxguest_linux.c
> > +++ b/drivers/virt/vboxguest/vboxguest_linux.c
> > @@ -88,8 +88,10 @@ static int vbg_misc_device_user_open(struct inode *inode, struct file *filp)
> >    */
> >   static int vbg_misc_device_close(struct inode *inode, struct file *filp)
> >   {
> > -     vbg_core_close_session(filp->private_data);
> > -     filp->private_data = NULL;
> > +     if (file->private_data != NULL) {
> > +             vbg_core_close_session(filp->private_data);
> > +             filp->private_data = NULL;
> > +     }
> >       return 0;
> >   }
> >
> >



-- 
Best regards!

Young
-----------------------------------------------------------

Re: [PATCH] vboxguest: check for private_data before trying to close it.

[Hans de Goede]

Hi,

On 28-05-19 16:49, Yang Xiao wrote:
> Based on your explanation, file->private_data can not be NULL before
> call vbg_core_close_session method all the time, since
> file->private_data is always set.
> Am I right?

Yes.

Regards,

Hans



> 
> On Tue, May 28, 2019 at 9:19 PM Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Hi,
>>
>> On 28-05-19 14:47, Young Xiao wrote:
>>> vbg_misc_device_close doesn't check that filp->private_data is non-NULL
>>> before trying to close_session, where vbg_core_close_session uses pointer
>>> session whithout checking, too. That can cause an oops in certain error
>>> conditions that can occur on open or lookup before the private_data is set.
>>>
>>> This vulnerability is similar to CVE-2011-1771.
>>
>> How is this in anyway related to CVE-2011-1771 ????
>>
>> That CVE is about a filesystems lookup method including a direct open
>> of the file for performance reasons and if that direct open fails, doing
>> a fput, which is how it ends up with a file with private_date being NULL,
>> see:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=703016
>>
>> "the problem is that CIFS doesn't do O_DIRECT at all, so when you try to open a file with it you get back -EINVAL. CIFS can also do open on lookup in some cases. In that case, fput will be called on the filp, which has not yet had its private_data set."
>>
>> The vboxguest code on the other hand is not a filessystem and as such
>> does not have a dentry lookup method at all!
>>
>> It's close method is part of the file_operations struct for a character
>> device using the misc_device framework which guarantees that private_data
>> is always set.
>>
>> Regards,
>>
>> Hans
>>
>>
>>
>>
>>
>>>
>>> Signed-off-by: Young Xiao <92siuyang@gmail.com>
>>> ---
>>>    drivers/virt/vboxguest/vboxguest_linux.c | 6 ++++--
>>>    1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/virt/vboxguest/vboxguest_linux.c b/drivers/virt/vboxguest/vboxguest_linux.c
>>> index 6e8c0f1..b03c16f 100644
>>> --- a/drivers/virt/vboxguest/vboxguest_linux.c
>>> +++ b/drivers/virt/vboxguest/vboxguest_linux.c
>>> @@ -88,8 +88,10 @@ static int vbg_misc_device_user_open(struct inode *inode, struct file *filp)
>>>     */
>>>    static int vbg_misc_device_close(struct inode *inode, struct file *filp)
>>>    {
>>> -     vbg_core_close_session(filp->private_data);
>>> -     filp->private_data = NULL;
>>> +     if (file->private_data != NULL) {
>>> +             vbg_core_close_session(filp->private_data);
>>> +             filp->private_data = NULL;
>>> +     }
>>>        return 0;
>>>    }
>>>
>>>
> 
> 
>