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

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Date:   Tue, 28 May 2019 15:19:10 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Young Xiao <92siuyang@...il.com>, 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 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;
>   }
>   
>