[PATCH 4.4 63/63] n_tty: fix EXTPROC vs ICANON interaction with TIOCINQ (aka FIONREAD)

Greg Kroah-Hartman
2018-01-01T14:29:43Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 966031f340185eddd05affcf72b740549f056348 upstream.

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>

---
drivers/tty/n_tty.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1801,7 +1801,7 @@ static void n_tty_set_termios(struct tty
{
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)) {
@@ -2493,7 +2493,7 @@ static int n_tty_ioctl(struct tty_struct
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);