[PATCH] staging: Fix atomicity violation in get_serial_info()

[Qiu-ji Chen <chenqiuji666@gmail.com>]

Atomicity violation occurs during consecutive reads of the members of 
gb_tty. Consider a scenario where, because the consecutive reads of gb_tty
members are not protected by a lock, the value of gb_tty may still be 
changing during the read process. 

gb_tty->port.close_delay and gb_tty->port.closing_wait are updated
together, such as in the set_serial_info() function. If during the
read process, gb_tty->port.close_delay and gb_tty->port.closing_wait
are still being updated, it is possible that gb_tty->port.close_delay
is updated while gb_tty->port.closing_wait is not. In this case,
the code first reads gb_tty->port.close_delay and then
gb_tty->port.closing_wait. A new gb_tty->port.close_delay and an
old gb_tty->port.closing_wait could be read. Such values, whether
before or after the update, should not coexist as they represent an
intermediate state.

This could result in a mismatch of the values read for gb_tty->minor, 
gb_tty->port.close_delay, and gb_tty->port.closing_wait, which in turn 
could cause ss->close_delay and ss->closing_wait to be mismatched.

To address this issue, we have enclosed all sequential read operations of 
the gb_tty variable within a lock. This ensures that the value of gb_tty 
remains unchanged throughout the process, guaranteeing its validity.

This possible bug is found by an experimental static analysis tool
developed by our team. This tool analyzes the locking APIs
to extract function pairs that can be concurrently executed, and then
analyzes the instructions in the paired functions to identify possible
concurrency bugs including data races and atomicity violations.

Fixes: b71e571adaa5 ("staging: greybus: uart: fix TIOCSSERIAL jiffies conversions")
Cc: stable@vger.kernel.org
Signed-off-by: Qiu-ji Chen <chenqiuji666@gmail.com>
---
 drivers/staging/greybus/uart.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index cdf4ebb93b10..8cc18590d97b 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -595,12 +595,14 @@ static int get_serial_info(struct tty_struct *tty,
 {
 	struct gb_tty *gb_tty = tty->driver_data;
 
+	mutex_lock(&gb_tty->port.mutex);
 	ss->line = gb_tty->minor;
 	ss->close_delay = jiffies_to_msecs(gb_tty->port.close_delay) / 10;
 	ss->closing_wait =
 		gb_tty->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
 		ASYNC_CLOSING_WAIT_NONE :
 		jiffies_to_msecs(gb_tty->port.closing_wait) / 10;
+	mutex_unlock(&gb_tty->port.mutex);
 
 	return 0;
 }
-- 
2.34.1