[PATCH net v3] atm/fore200e: Fix possible data race in fore200e_open()

[Gui-Dong Han <hanguidong02@gmail.com>]

Protect access to fore200e->available_cell_rate with rate_mtx lock in the
error handling path of fore200e_open() to prevent a data race.

The field fore200e->available_cell_rate is a shared resource used to track
available bandwidth. It is concurrently accessed by fore200e_open(),
fore200e_close(), and fore200e_change_qos().

In fore200e_open(), the lock rate_mtx is correctly held when subtracting
vcc->qos.txtp.max_pcr from available_cell_rate to reserve bandwidth.
However, if the subsequent call to fore200e_activate_vcin() fails, the
function restores the reserved bandwidth by adding back to
available_cell_rate without holding the lock.

This introduces a race condition because available_cell_rate is a global
device resource shared across all VCCs. If the error path in
fore200e_open() executes concurrently with operations like
fore200e_close() or fore200e_change_qos() on other VCCs, a
read-modify-write race occurs.

Specifically, the error path reads the rate without the lock. If another
CPU acquires the lock and modifies the rate (e.g., releasing bandwidth in
fore200e_close()) between this read and the subsequent write, the error
path will overwrite the concurrent update with a stale value. This results
in incorrect bandwidth accounting.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
---
v3:
* Expanded the commit message to describe the specific call paths causing
the race, as suggested by Jakub Kicinski and Paolo Abeni.
v2:
* Added a description of the data race hazard in fore200e_open(), as
suggested by Jakub Kicinski and Simon Horman.
---
 drivers/atm/fore200e.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/atm/fore200e.c b/drivers/atm/fore200e.c
index 4fea1149e003..f62e38571440 100644
--- a/drivers/atm/fore200e.c
+++ b/drivers/atm/fore200e.c
@@ -1374,7 +1374,9 @@ fore200e_open(struct atm_vcc *vcc)
 
 	vcc->dev_data = NULL;
 
+	mutex_lock(&fore200e->rate_mtx);
 	fore200e->available_cell_rate += vcc->qos.txtp.max_pcr;
+	mutex_unlock(&fore200e->rate_mtx);
 
 	kfree(fore200e_vcc);
 	return -EINVAL;
-- 
2.34.1