linux-kernel - Re: [regression] [bisected] commit 6bb6fa6908ebd3cb4e14cd4f0ce272ec885d2eb0 corrupts data sent via pseudoterminal device

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Date: Tue, 14 May 2024 14:03:05 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>, 
    Vadym Krevs <vkrevs@...oo.com>
cc: Bagas Sanjaya <bagasdotme@...il.com>, 
    Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
    Linux Regressions <regressions@...ts.linux.dev>, 
    Linux Serial <linux-serial@...r.kernel.org>, 
    Gilles Buloz <gilles.buloz@...tron.com>, 
    Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [regression] [bisected] commit 6bb6fa6908ebd3cb4e14cd4f0ce272ec885d2eb0
 corrupts data sent via pseudoterminal device

On Tue, 14 May 2024, Andy Shevchenko wrote:

> On Tue, May 14, 2024 at 12:28 PM Vadym Krevs <vkrevs@...oo.com> wrote:
> >
> > It's a standard setup for an out-of-the box default install of openSUSE 15.5 with KDE. All tests done in Konsole with bash as shell.
> >
> > stty -a -F /dev/pts/1
> > speed 38400 baud; rows 57; columns 217; line = 0;
> > intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
> > -parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
> > -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon ixoff -iuclc -ixany -imaxbel iutf8
> > opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
> > isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc
> 
> Thank you!
> 
> Yeah. SW flow control is enabled, but I don't see which character is
> being used for that. Anyway, let's give Ilpo a chance to look into
> this.

Thanks a lot for pinpointing the commit with bisect. It turns out this 
is a quite bad corruption bug and I'm quite surprised I didn't see (or 
notice) it while testing the patch.

Could you please test and confirm the patch below fixes the issue?

--
[PATCH] tty: n_tty: Fix buffer offsets when looked ahead is used

When lookahead has "consumed" some characters (la_count > 0),
n_tty_receive_buf_standard() and n_tty_receive_buf_closing() for
characters beyond the la_count are given wrong cp/fp offsets which
leads to duplicating and losing some characters.

If la_count > 0, correct buffer pointers and make count consistency too
(the latter is not strictly necessary to fix the issue but seems more
logical to adjust all variables immediately to keep state consistent).

Reported-by: Vadym Krevs <vkrevs@...oo.com>
Fixes: 6bb6fa6908eb ("tty: Implement lookahead to process XON/XOFF timely")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218834
Cc: stable@...r.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/tty/n_tty.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index f252d0b5a434..5e9ca4376d68 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1619,15 +1619,25 @@ static void __receive_buf(struct tty_struct *tty, const u8 *cp, const u8 *fp,
 	else if (ldata->raw || (L_EXTPROC(tty) && !preops))
 		n_tty_receive_buf_raw(tty, cp, fp, count);
 	else if (tty->closing && !L_EXTPROC(tty)) {
-		if (la_count > 0)
+		if (la_count > 0) {
 			n_tty_receive_buf_closing(tty, cp, fp, la_count, true);
-		if (count > la_count)
-			n_tty_receive_buf_closing(tty, cp, fp, count - la_count, false);
+			cp += la_count;
+			if (fp)
+				fp += la_count;
+			count -= la_count;
+		}
+		if (count > 0)
+			n_tty_receive_buf_closing(tty, cp, fp, count, false);
 	} else {
-		if (la_count > 0)
+		if (la_count > 0) {
 			n_tty_receive_buf_standard(tty, cp, fp, la_count, true);
-		if (count > la_count)
-			n_tty_receive_buf_standard(tty, cp, fp, count - la_count, false);
+			cp += la_count;
+			if (fp)
+				fp += la_count;
+			count -= la_count;
+		}
+		if (count > 0)
+			n_tty_receive_buf_standard(tty, cp, fp, count, false);
 
 		flush_echoes(tty);
 		if (tty->ops->flush_chars)
-- 
2.39.2