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

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 <redacted>
---
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