linux-kernel - Re: [PATCH] vboxguest: check for private

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Date:   Tue, 28 May 2019 16:51:22 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Yang Xiao <92siuyang@...il.com>
Cc:     arnd@...db.de, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vboxguest: check for private_data before trying to close
 it.

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@...hat.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@...il.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;
>>>    }
>>>
>>>
> 
> 
>