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commitb2f37aead1upstream. The previous commit3e0588c291("hamradio: defer ax25 kfree after unregister_netdev") reorder the kfree operations and unregister_netdev operation to prevent UAF. This commit improves the previous one by also deferring the nullify of the ax->tty pointer. Otherwise, a NULL pointer dereference bug occurs. Partial of the stack trace is shown below. BUG: kernel NULL pointer dereference, address: 0000000000000538 RIP: 0010:ax_xmit+0x1f9/0x400 ... Call Trace: dev_hard_start_xmit+0xec/0x320 sch_direct_xmit+0xea/0x240 __qdisc_run+0x166/0x5c0 __dev_queue_xmit+0x2c7/0xaf0 ax25_std_establish_data_link+0x59/0x60 ax25_connect+0x3a0/0x500 ? security_socket_connect+0x2b/0x40 __sys_connect+0x96/0xc0 ? __hrtimer_init+0xc0/0xc0 ? common_nsleep+0x2e/0x50 ? switch_fpu_return+0x139/0x1a0 __x64_sys_connect+0x11/0x20 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 The crash point is shown as below static void ax_encaps(...) { ... set_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags); // ax->tty = NULL! ... } By placing the nullify action after the unregister_netdev, the ax->tty pointer won't be assigned as NULL net_device framework layer is well synchronized. Signed-off-by: Lin Ma <linma@zju.edu.cn> Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.