n_tty: fix EXTPROC vs ICANON interaction with TIOCINQ (aka FIONREAD)

Orabug: 28855335

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>
(cherry picked from commit 966031f340185eddd05affcf72b740549f056348)
CVE: CVE-2018-18386
Signed-off-by: Dan Duval <dan.duval@oracle.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index aba20f66bdd9e85ee6d29599791ca82d0578a7b2..66e257b5a5b75fd4b7d465bd04aaaa81f2fc60e8 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1808,7 +1808,7 @@ 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)) {
@@ -2517,7 +2517,7 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
                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);