CVE-2018-18386: Incorrect Type Conversion or Cast (CWE-704)
value: drivers/tty/n_tty.c in the Linux kernel before 4.14.11 allows local attackers (who are able to access pseudo terminals) to hang/block further usage of any pseudo terminal devices due to an EXTPROC versus ICANON confusion in TIOCINQ.
CWE description: The product does not correctly convert an object, resource, or structure from one type to a different type.
CVSS v3 base score: 3.3
Published: 2018-10-17T20:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: drivers/tty/n_tty.c
Commit date: 2017-12-21 11:19:22 +0100
Commit message: n_tty: fix EXTPROC vs ICANON interaction with TIOCINQ (aka FIONREAD)

We added support for EXTPROC back in 2010 in commit 26df6d13406d ("tty:
Add EXTPROC support for LINEMODE") and the intent was to allow it to
override some (all?) ICANON behavior.  Quoting from that original commit
message:

         There is a new bit in the termios local flag word, EXTPROC.
         When this bit is set, several aspects of the terminal driver
         are disabled.  Input line editing, character echo, and mapping
         of signals are all disabled.  This allows the telnetd to turn
         off these functions when in linemode, but still keep track of
         what state the user wants the terminal to be in.

but the problem turns out that "several aspects of the terminal driver
are disabled" is a bit ambiguous, and you can really confuse the n_tty
layer by setting EXTPROC and then causing some of the ICANON invariants
to no longer be maintained.

This fixes at least one such case (TIOCINQ) becoming unhappy because of
the confusion over whether ICANON really means ICANON when EXTPROC is set.

This basically makes TIOCINQ match the case of read: if EXTPROC is set,
we ignore ICANON.  Also, make sure to reset the ICANON state ie EXTPROC
changes, not just if ICANON changes.

Fixes: 26df6d13406d ("tty: Add EXTPROC support for LINEMODE")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Reported-by: syzkaller <syzkaller@googlegroups.com>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Diff: diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 427e0d5d8f13..539b49adb6af 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1760,11 +1760,11 @@ static int n_tty_receive_buf2(struct tty_struct *tty, const unsigned char *cp,
 
 static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	if (!old || (old->c_lflag ^ tty->termios.c_lflag) & ICANON) {
+	if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
 		bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
 		ldata->line_start = ldata->read_tail;
 		if (!L_ICANON(tty) || !read_cnt(ldata)) {
 			ldata->canon_head = ldata->read_tail;
 			ldata->push = 0;
@@ -2423,11 +2423,11 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
 	switch (cmd) {
 	case TIOCOUTQ:
 		return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
 	case TIOCINQ:
 		down_write(&tty->termios_rwsem);
-		if (L_ICANON(tty))
+		if (L_ICANON(tty) && !L_EXTPROC(tty))
 			retval = inq_canon(ldata);
 		else
 			retval = read_cnt(ldata);
 		up_write(&tty->termios_rwsem);
 		return put_user(retval, (unsigned int __user *) arg);

Vulnerable code: 	if (!old || (old->c_lflag ^ tty->termios.c_lflag) & ICANON) {
		if (L_ICANON(tty))
Fixed code: 	if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
		if (L_ICANON(tty) && !L_EXTPROC(tty))