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

From: Anirudh Rayabharam <redacted>
Date: Tue,  6 Apr 2021 18:13:59 +0530

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@syzkaller.appspotmail.com
Tested-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com

Signed-off-by: Anirudh Rayabharam <redacted>

This adds a new build warning:

  CC [M]  drivers/net/usb/hso.o
drivers/net/usb/hso.c: In function ‘hso_serial_common_create’:
drivers/net/usb/hso.c:2256:6: warning: unused variable ‘minor’ [-Wunused-variable]

Please fix this and add an appropriate Fixes: tag, thank you.