Re: [PATCH] tty: n_gsm: fix missing assignment of gsm->receive() in gsmld_attach_gsm()

From: Greg KH
Date: Wed Aug 10 2022 - 04:50:10 EST


On Wed, Aug 10, 2022 at 06:12:52AM +0300, Mazin Al Haddad wrote:
> Fixes a bug reported by syzbot.

I do not understand this sentence :(

> A null pointer dereference can
> happen when attempting to access the "gsm->receive()" function in
> gsmld_receive_buf(). Currently, the "gsm->receive()" function is only set
> after a call to the GSMIO_SETCONF ioctl. Since the gsmld_receive_buf()
> function can be accessed without the need to call the line discipline
> ioctl (GSMIO_SETCONF), the gsm->receive() function will not be set and a
> NULL pointer dereference will occur.
>
> Fix this by setting the gsm->receive() function when the line discipline
> is being attached to the terminal device, inside gsmld_attach_gsm(). This
> will guarantee that the function is assigned and a call to TIOCSTI,
> which calls gsmld_receive_buf(), will not reference a null pointer.
>
> Call Trace:
> <TASK>
> gsmld_receive_buf+0x1c2/0x2f0 drivers/tty/n_gsm.c:2861
> tiocsti drivers/tty/tty_io.c:2293 [inline]
> tty_ioctl+0xa75/0x15d0 drivers/tty/tty_io.c:2692
> vfs_ioctl fs/ioctl.c:51 [inline]
> __do_sys_ioctl fs/ioctl.c:870 [inline]
> __se_sys_ioctl fs/ioctl.c:856 [inline]
> __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
>
> Fixes: 01aecd917114 ("tty: n_gsm: fix tty registration before control channel open")
> Reported-and-tested-by: syzbot+e3563f0c94e188366dbb@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Mazin Al Haddad <mazinalhaddad05@xxxxxxxxx>

What commit does this fix?

thanks,

greg k-h