Re: [PATCH] Fix a double free bug in rsi_91x_deinit

From: Guenter Roeck
Date: Mon Sep 02 2019 - 15:32:47 EST

On 9/2/19 11:47 AM, Greg KH wrote:
On Sun, Sep 01, 2019 at 07:08:29AM -0700, Guenter Roeck wrote:
On 9/1/19 1:03 AM, Kalle Valo wrote:
Guenter Roeck <linux@xxxxxxxxxxxx> writes:

On Mon, Aug 19, 2019 at 06:02:29PM -0400, Hui Peng wrote:
`dev` (struct rsi_91x_usbdev *) field of adapter
(struct rsi_91x_usbdev *) is allocated and initialized in
`rsi_init_usb_interface`. If any error is detected in information
read from the device side, `rsi_init_usb_interface` will be
freed. However, in the higher level error handling code in
`rsi_probe`, if error is detected, `rsi_91x_deinit` is called
again, in which `dev` will be freed again, resulting double free.

This patch fixes the double free by removing the free operation on
`dev` in `rsi_init_usb_interface`, because `rsi_91x_deinit` is also
used in `rsi_disconnect`, in that code path, the `dev` field is not
(and thus needs to be) freed.

This bug was found in v4.19, but is also present in the latest version
of kernel.

Reported-by: Hui Peng <benquike@xxxxxxxxx>
Reported-by: Mathias Payer <mathias.payer@xxxxxxxxxxxxx>
Signed-off-by: Hui Peng <benquike@xxxxxxxxx>


Reviewed-by: Guenter Roeck <linux@xxxxxxxxxxxx>

This patch is listed as fix for CVE-2019-15504, which has a CVSS 2.0 score
of 10.0 (high) and CVSS 3.0 score of 9.8 (critical).

A double free in error path is considered as a critical CVE issue? I'm
very curious, why is that?

You'd have to ask the people assigning CVSS scores. However, if the memory
was reallocated, that reallocated memory (which is still in use) is freed.
Then all kinds of bad things can happen.

Yes, but moving from "bad things _can_ happen" to "bad things happen" in
an instance like this will be a tough task. It also requires physical
access to the machine.

Is this correct even with usbip enabled ?

Anyway, that doesn't mean we shouldn't fix it, it's just that CVSS can
be crazy when it comes to kernel patches (i.e. almost all fixes should
be "critical"...)

Not all of them, but probably too many. That is why I asked if the problem
is real. I _used_ to trust CVSS scores, but by now I am at least somewhat
suspicious - especially if a patch wasn't applied for a period of time,
like this series of usb patches.

Having said that, I am even more wary of double-free problems - those tend
to be notoriously difficult to debug. I'd rather have them out of my way,
even if they are unlikely to be seen in the real world (plus, Murphy
says that anything unlikely is going to happen almost immediately).