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

[Mazin Al Haddad]

Fixes a bug reported by syzbot. 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>
---
 drivers/tty/n_gsm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index caa5c14ed57f..1d74dd9d70d0 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2825,6 +2825,11 @@ static void gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	/* Turn off tty XON/XOFF handling to handle it explicitly. */
 	gsm->old_c_iflag = tty->termios.c_iflag;
 	tty->termios.c_iflag &= (IXON | IXOFF);
+
+	if (gsm->encoding == 0)
+		gsm->receive = gsm0_receive;
+	else
+		gsm->receive = gsm1_receive;
 }
 
 /**
-- 
2.37.1