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@xxxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >    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;
> > >    }