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([41.36.52.230]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4834d7e50casm141590125e9.8.2026.02.11.13.08.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Feb 2026 13:08:43 -0800 (PST) From: Osama Abdelkader To: Greg Kroah-Hartman , Jiri Slaby , Andy Shevchenko , =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= , linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Cc: Osama Abdelkader , syzbot+80806cf7508e92c7cc86@syzkaller.appspotmail.com Subject: [PATCH] tty: n_tty: fix KCSAN data-race in n_tty_flush_buffer / n_tty_lookahead_flow_ctrl Date: Wed, 11 Feb 2026 22:08:38 +0100 Message-ID: <20260211210838.45127-1-osama.abdelkader@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" n_tty_lookahead_flow_ctrl() accesses ldata->lookahead_count without holding termios_rwsem, while reset_buffer_flags() in n_tty_flush_buffer() resets it with exclusive termios_rwsem held. This causes a data race reported by KCSAN when a PTY is closed while flush_to_ldisc is still processing lookahead data. Fix by taking termios_rwsem (read) in n_tty_lookahead_flow_ctrl(), consistent with __receive_buf() which also modifies lookahead_count under the read lock. Reported-by: syzbot+80806cf7508e92c7cc86@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3D80806cf7508e92c7cc86 Fixes: 6bb6fa6908eb ("tty: Implement lookahead to process XON/XOFF timely") Signed-off-by: Osama Abdelkader --- drivers/tty/n_tty.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index e6a0f5b40d0a..725d6ed2b863 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -1480,6 +1480,8 @@ static void n_tty_lookahead_flow_ctrl(struct tty_stru= ct *tty, const u8 *cp, struct n_tty_data *ldata =3D tty->disc_data; u8 flag =3D TTY_NORMAL; =20 + guard(rwsem_read)(&tty->termios_rwsem); + ldata->lookahead_count +=3D count; =20 if (!I_IXON(tty)) --=20 2.43.0