net: hso: fix null-ptr-deref during tty device unregistration

stable inclusion
from stable-v4.19.187
commit 92028d7a31e55d53e41cff679156b9432cffcb36
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I93LMH
CVE: CVE-2021-46904

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92028d7a31e55d53e41cff679156b9432cffcb36



---------------------------

commit 8a12f883 upstream.

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.

Fixes: 72dc1c09 ("HSO: add option hso driver")
Reported-by:  <syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com>
Tested-by:  <syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Liu Jian <liujian56@huawei.com>