Re: [PATCH] net: hso: fix null-ptr-deref during tty device unregistration

[Greg KH]

On Tue, Apr 06, 2021 at 06:13:59PM +0530, Anirudh Rayabharam wrote:
> Multiple ttys try to claim the same the minor number causing a double
> unregistration of the same device. The first unregistration succeeds
> but the next one results in a null-ptr-deref.
> 
> The get_free_serial_index() function returns an available minor number
> but doesn't assign it immediately. The assignment is done by the caller
> later. But before this assignment, calls to get_free_serial_index()
> would return the same minor number.
> 
> Fix this by modifying get_free_serial_index to assign the minor number
> immediately after one is found to be and rename it to obtain_minor()
> to better reflect what it does. Similary, rename set_serial_by_index()
> to release_minor() and modify it to free up the minor number of the
> given hso_serial. Every obtain_minor() should have corresponding
> release_minor() call.
> 
> Reported-by: syzbot+c49fe6089f295a05e6f8@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: syzbot+c49fe6089f295a05e6f8@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> Signed-off-by: Anirudh Rayabharam <mail@xxxxxxxxxxxxx>

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>