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[34.82.187.25]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2e2e5c4a240sm160065ad.81.2026.09.29.21.12.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Sep 2026 21:12:19 -0700 (PDT) From: Hui Peng To: Greg Kroah-Hartman , Jiri Slaby , John Ogness , =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= Cc: Andy Shevchenko , linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Hui Peng Subject: [PATCH v5 2/3] serial: core: allow third iteration in uart_get_baud_rate() fallback Date: Wed, 30 Sep 2026 04:12:15 +0000 Message-ID: <20260930041216.155911-3-benquike@gmail.com> X-Mailer: git-send-email 2.56.0.rc1.315.gc6ed9934b7-goog In-Reply-To: <20260930041216.155911-1-benquike@gmail.com> References: <20260930041216.155911-1-benquike@gmail.com> Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit In uart_get_baud_rate(), when the requested baud rate is out of range (try == 0) and old is non-NULL, the function copies the old termios baud rate into termios, sets old = NULL, and continues to try == 1. If the old baud rate is also out of range (for instance, after uport->uartclk was lowered via TIOCSSERIAL so max = uport->uartclk / 16 is smaller than both the old and new baud rates), try == 1 clips baud to [min + 1, max - 1] and encodes it into termios via tty_termios_encode_baud_rate(). However, because the loop bound is for (try = 0; try < 2; try++), the loop terminates immediately after try == 1 without re-evaluating baud = tty_termios_baud_rate(termios) for the clipped rate, hitting WARN_ON(1) and returning 0, which then triggers a fatal divide-by-zero (Oops: divide error) in uart_get_divisor(): WARNING: drivers/tty/serial/serial_core.c:548 at uart_get_baud_rate+0x136/0x260 Oops: divide error: 0000 [#1] SMP KASAN PTI RIP: 0010:uart_get_divisor+0x5b/0x100 Call Trace: serial8250_get_divisor+0x123/0x1a0 serial8250_do_set_termios+0x21e/0x1610 tty_ioctl+0x1028/0x1480 Increase the loop limit to try < 3 so that the clipped baud rate encoded on try == 1 is evaluated and returned on try == 2. Tested in QEMU against Linux 7.3.0-rc3 by setting /dev/ttyS1 to B115200, lowering baud_base to 9600 (max = 9600) via TIOCSSERIAL, and calling tcsetattr() with B57600 (old = B115200), reproducing the WARNING and Oops: divide error in uart_get_divisor() on the unfixed kernel and verifying clean execution with 0 warnings/faults with the fix applied. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Fixes: 091ea8e5d34e ("serial: core: prevent division by zero by always returning non-zero baud rate") Cc: stable@vger.kernel.org Reviewed-by: Ilpo Järvinen Assisted-by: LLM Signed-off-by: Hui Peng --- Changes in v5: - Trim kernel stack trace in commit message to 4 key lines, as suggested by Andy Shevchenko. Changes in v4: - No changes. Changes in v3: - Collect Reviewed-by from Ilpo Järvinen. Changes in v2: - Split out from the 8250_core.c and uart_set_info() changes into patch 2/3, add Cc: stable@vger.kernel.org, and include the QEMU reproducer test details and kernel stack trace, as requested by Greg Kroah-Hartman. drivers/tty/serial/serial_core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 95774b0f1484..3845fb582f45 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -501,7 +501,7 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios, break; } - for (try = 0; try < 2; try++) { + for (try = 0; try < 3; try++) { baud = tty_termios_baud_rate(termios); /*