git: 8647fe60b8c3 - stable/13 - tty: properly check character position when handling IUTF8 backspaces

The branch stable/13 has been updated by christos:

URL: https://cgit.FreeBSD.org/src/commit/?id=8647fe60b8c3cc03cf1d53d281f9306b7043543b

commit 8647fe60b8c3cc03cf1d53d281f9306b7043543b
Author:     Bojan Novković <bojan.novkovic@fer.hr>
AuthorDate: 2023-11-13 18:02:30 +0000
Commit:     Christos Margiolis <christos@FreeBSD.org>
CommitDate: 2023-11-20 16:57:49 +0000

    tty: properly check character position when handling IUTF8 backspaces
    
    The tty_rubchar() code handling backspaces for UTF-8 characters didn't
    properly check whether the beginning of the current line was reached.
    This resulted in a kernel panic in ttyinq_unputchar() when prodded with
    certain malformed UTF-8 sequences.
    
    PR:             275009
    Reviewed by:    christos
    Differential Revision:  https://reviews.freebsd.org/D42564
    
    (cherry picked from commit c6d7be214811c315d234d64c6cbaa92d4f55d2c1)
---
 sys/kern/tty_ttydisc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/sys/kern/tty_ttydisc.c b/sys/kern/tty_ttydisc.c
index c46579cefbb1..cb5bf672d040 100644
--- a/sys/kern/tty_ttydisc.c
+++ b/sys/kern/tty_ttydisc.c
@@ -822,7 +822,13 @@ ttydisc_rubchar(struct tty *tp)
 				/* Loop back through inq until we hit the
 				 * leading byte. */
 				while (CTL_UTF8_CONT(c) && nb < UTF8_STACKBUF) {
-					ttyinq_peekchar(&tp->t_inq, &c, &quote);
+					/*
+					 * Check if we've reached the beginning
+					 * of the line.
+					 */
+					if (ttyinq_peekchar(&tp->t_inq, &c,
+					    &quote) != 0)
+						break;
 					ttyinq_unputchar(&tp->t_inq);
 					bytes[curidx] = c;
 					curidx--;