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

From: Hans de Goede
Date: Tue May 28 2019 - 09:22:35 EST


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,

"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.



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;