[PATCH v3 0/3] serial: fix IRQ chain race, baud fallback, and uartclk overflow

[PATCH v3 0/3] serial: fix IRQ chain race, baud fallback, and uartclk overflow

[Hui Peng]

This series fixes three bugs in the 8250 and serial core drivers:

1. Hold hash_mutex across IRQ chain lookup, linking, and error-path
   unlinking in serial_link_irq_chain() to prevent a use-after-free race
   with concurrent serial_unlink_irq_chain().
2. Allow a third loop iteration (try < 3) in uart_get_baud_rate() so that
   when both the requested and old baud rates are out of range and try == 1
   clips the rate into termios, the clipped baud rate is evaluated and
   returned on try == 2 instead of returning 0 and causing a divide-by-zero
   in uart_get_divisor().
3. Reject baud_base > UINT_MAX / 16 in uart_set_info() alongside the
   uartclk == 0 check so that multiplying baud_base by 16 cannot wrap
   around in 32-bit unsigned arithmetic to a small non-zero uartclk.

Changes in v3:
- Patch 2/3: Collect Reviewed-by from Ilpo Järvinen.
- Patch 3/3: Add missing #include <linux/limits.h> for UINT_MAX and move
  the new_info->baud_base > UINT_MAX / 16 check to the uartclk == 0 check,
  as suggested by Ilpo Järvinen.

Changes in v2:
- Split the 8250_core.c and serial_core.c fixes into a 3-patch series,
  add Cc: stable@vger.kernel.org, and document how each patch was tested in
  QEMU, as requested by Greg Kroah-Hartman and John Ogness.

Hui Peng (3):
  serial: 8250: hold hash_mutex across IRQ chain linking in
    serial_link_irq_chain()
  serial: core: allow third iteration in uart_get_baud_rate() fallback
  serial: core: reject baud_base values that overflow port->uartclk in
    uart_set_info()

 drivers/tty/serial/8250/8250_core.c | 4 ++--
 drivers/tty/serial/serial_core.c    | 5 +++--
 2 files changed, 5 insertions(+), 4 deletions(-)

-- 
2.49.0

[PATCH v3 1/3] serial: 8250: hold hash_mutex across IRQ chain linking in serial_link_irq_chain()

[Hui Peng]

In serial_link_irq_chain(), serial_get_or_create_irq_info() acquires and
releases hash_mutex before returning struct irq_info *i to the caller.
Before serial_link_irq_chain() links the port into i->head, a concurrent
serial_unlink_irq_chain() on the same shared IRQ line can observe a
single-port i->head under hash_mutex, remove i from irq_lists, and
kfree(i), causing a use-after-free when serial_link_irq_chain() accesses
i->lock and i->head. In addition, if request_irq() fails at the end of
serial_link_irq_chain(), serial_do_unlink(i, up) calls hlist_del(&i->node)
and kfree(i) without holding hash_mutex.

Move guard(mutex)(&hash_mutex) from serial_get_or_create_irq_info() to its
sole caller serial_link_irq_chain() so that hash_mutex is held across the
lookup/allocation of struct irq_info, the insertion into i->head, and any
error-path serial_do_unlink() cleanup.

Tested in QEMU against Linux 7.3.0-rc3 by configuring /dev/ttyS1 and
/dev/ttyS2 to share IRQ 3 with ASYNC_SHARE_IRQ via TIOCSSERIAL and
concurrently opening and closing both ports from two threads in a tight
loop with KASAN enabled, verifying 0 KASAN faults or warnings.

Fixes: 25db8ad5c567 ("serial, 8250: remove NR_IRQ usage")
Fixes: 99fc860fae83 ("serial: 8250: extract serial_get_or_create_irq_info()")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v3:
- No changes.

Changes in v2:
- Split the 8250_core.c and serial_core.c fixes into a 3-patch series,
  add Cc: stable@vger.kernel.org, and document how the patch was tested,
  as requested by Greg Kroah-Hartman.

 drivers/tty/serial/8250/8250_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index b875d394796f..0bc810f285a4 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -134,8 +134,6 @@ static struct irq_info *serial_get_or_create_irq_info(const struct uart_8250_por
 {
 	struct irq_info *i;
 
-	guard(mutex)(&hash_mutex);
-
 	hash_for_each_possible(irq_lists, i, node, up->port.irq)
 		if (i->irq == up->port.irq)
 			return i;
@@ -156,6 +154,8 @@ static int serial_link_irq_chain(struct uart_8250_port *up)
 	struct irq_info *i;
 	int ret;
 
+	guard(mutex)(&hash_mutex);
+
 	i = serial_get_or_create_irq_info(up);
 	if (IS_ERR(i))
 		return PTR_ERR(i);
-- 
2.49.0

[PATCH v3 2/3] serial: core: allow third iteration in uart_get_baud_rate() fallback

[Hui Peng]

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, CPU#1: init/1
  ...
  Oops: divide error: 0000 [#1] SMP KASAN PTI
  CPU: 1 UID: 0 PID: 1 Comm: init Tainted: G        W        N  7.3.0-rc3-g5dd1818b15d9 #1 PREEMPT(lazy)
  RIP: 0010:uart_get_divisor+0x5b/0x100
  Call Trace:
   <TASK>
   serial8250_get_divisor+0x123/0x1a0
   serial8250_do_set_termios+0x21e/0x1610
   serial8250_set_termios+0x77/0x90
   uart_change_line_settings+0xf6/0x720
   uart_set_termios+0x1c1/0x5b0
   tty_set_termios+0x5f7/0x920
   set_termios+0x533/0x7d0
   tty_mode_ioctl+0x8e4/0xd10
   n_tty_ioctl_helper+0x3c/0x270
   n_tty_ioctl+0x4e/0x2c0
   tty_ioctl+0x1028/0x1480
   __x64_sys_ioctl+0x184/0x1d0
   do_syscall_64+0xda/0x4b0

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 <ilpo.jarvinen@linux.intel.com>
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
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);
 
 		/*
-- 
2.49.0

[PATCH v3 3/3] serial: core: reject baud_base values that overflow port->uartclk in uart_set_info()

[Hui Peng]

In uart_set_info(), new_info->baud_base is multiplied by 16 and stored in
uport->uartclk (an unsigned int):

  uport->uartclk = new_info->baud_base * 16;

While uart_set_info() checks if (uartclk == 0) and
if (new_info->baud_base < 9600), when new_info->baud_base exceeds
UINT_MAX / 16 with low bits set (for example, 0x10000001), multiplying
by 16 wraps around in 32-bit unsigned arithmetic to a small non-zero value
(16), bypassing both uartclk == 0 and new_info->baud_base < 9600 and
setting uport->uartclk = 16 (baud_base = 1, well below the required
minimum of 9600 * 16).

Reject new_info->baud_base > UINT_MAX / 16 alongside the uartclk == 0
check in uart_set_info().

Tested in QEMU against Linux 7.3.0-rc3 by calling ioctl(fd, TIOCSSERIAL,
&ss) with ss.baud_base = 0x10000001 on /dev/ttyS1: on the unfixed kernel
TIOCSSERIAL succeeds (ret = 0) and wraps uport->uartclk to 16
(TIOCGSERIAL reports baud_base = 1), whereas with the fix applied
TIOCSSERIAL returns -EINVAL and preserves the existing uport->uartclk.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 6eabce6608d6 ("serial: core: check uartclk for zero to avoid divide by zero")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v3:
- Add missing #include <linux/limits.h> for UINT_MAX and move the
  new_info->baud_base > UINT_MAX / 16 check to the uartclk == 0 check, as
  suggested by Ilpo Järvinen.

Changes in v2:
- Split out into patch 3/3 of the series, add Cc: stable@vger.kernel.org,
  and document the QEMU test method, as requested by Greg Kroah-Hartman.

 drivers/tty/serial/serial_core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 3845fb582f45..362dd6648ba4 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -16,6 +16,7 @@
 #include <linux/console.h>
 #include <linux/gpio/consumer.h>
 #include <linux/kernel.h>
+#include <linux/limits.h>
 #include <linux/of.h>
 #include <linux/pm_runtime.h>
 #include <linux/proc_fs.h>
@@ -933,7 +934,7 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 	if (!(uport->flags & UPF_FIXED_PORT)) {
 		unsigned int uartclk = new_info->baud_base * 16;
 		/* check needs to be done here before other settings made */
-		if (uartclk == 0)
+		if (uartclk == 0 || new_info->baud_base > UINT_MAX / 16)
 			return -EINVAL;
 	}
 	if (!capable(CAP_SYS_ADMIN)) {
-- 
2.49.0

Re: [PATCH v3 3/3] serial: core: reject baud_base values that overflow port->uartclk in uart_set_info()

[Ilpo Järvinen]

[-- Attachment #1: Type: text/plain, Size: 2861 bytes --]

On Tue, 22 Sep 2026, Hui Peng wrote:

> In uart_set_info(), new_info->baud_base is multiplied by 16 and stored in
> uport->uartclk (an unsigned int):
> 
>   uport->uartclk = new_info->baud_base * 16;
> 
> While uart_set_info() checks if (uartclk == 0) and
> if (new_info->baud_base < 9600), when new_info->baud_base exceeds
> UINT_MAX / 16 with low bits set (for example, 0x10000001), multiplying
> by 16 wraps around in 32-bit unsigned arithmetic to a small non-zero value
> (16), bypassing both uartclk == 0 and new_info->baud_base < 9600 and
> setting uport->uartclk = 16 (baud_base = 1, well below the required
> minimum of 9600 * 16).
> 
> Reject new_info->baud_base > UINT_MAX / 16 alongside the uartclk == 0
> check in uart_set_info().
> 
> Tested in QEMU against Linux 7.3.0-rc3 by calling ioctl(fd, TIOCSSERIAL,
> &ss) with ss.baud_base = 0x10000001 on /dev/ttyS1: on the unfixed kernel
> TIOCSSERIAL succeeds (ret = 0) and wraps uport->uartclk to 16
> (TIOCGSERIAL reports baud_base = 1), whereas with the fix applied
> TIOCSSERIAL returns -EINVAL and preserves the existing uport->uartclk.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: 6eabce6608d6 ("serial: core: check uartclk for zero to avoid divide by zero")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v3:
> - Add missing #include <linux/limits.h> for UINT_MAX and move the
>   new_info->baud_base > UINT_MAX / 16 check to the uartclk == 0 check, as
>   suggested by Ilpo Järvinen.
> 
> Changes in v2:
> - Split out into patch 3/3 of the series, add Cc: stable@vger.kernel.org,
>   and document the QEMU test method, as requested by Greg Kroah-Hartman.
> 
>  drivers/tty/serial/serial_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 3845fb582f45..362dd6648ba4 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -16,6 +16,7 @@
>  #include <linux/console.h>
>  #include <linux/gpio/consumer.h>
>  #include <linux/kernel.h>
> +#include <linux/limits.h>
>  #include <linux/of.h>
>  #include <linux/pm_runtime.h>
>  #include <linux/proc_fs.h>
> @@ -933,7 +934,7 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
>  	if (!(uport->flags & UPF_FIXED_PORT)) {
>  		unsigned int uartclk = new_info->baud_base * 16;
>  		/* check needs to be done here before other settings made */
> -		if (uartclk == 0)
> +		if (uartclk == 0 || new_info->baud_base > UINT_MAX / 16)

Logically it would make more sense to check the overflow before 
multiplying even if this order too produces the "correct" result.

>  			return -EINVAL;
>  	}
>  	if (!capable(CAP_SYS_ADMIN)) {
> 

-- 
 i.