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

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Date:   Tue, 28 May 2019 22:49:04 +0800
From:   Yang Xiao <92siuyang@...il.com>
To:     Hans de Goede <hdegoede@...hat.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.

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@...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;
> >   }
> >
> >



-- 
Best regards!

Young
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