Re: [PATCH] hwmon: (corsair-psu) serialize debugfs access against hwmon

[Wilken Gottwalt <wilken.gottwalt@posteo.net>]

On Sun,  2 Aug 2026 12:36:53 +0000
Ali Ahmet Memis <ali@iusegentoo.com> wrote:

> corsairpsu_request() sends a rail select command and then the actual
> read as two separate transfers, both going through the single shared
> cmd_buffer and wait_completion in corsairpsu_usb_cmd(). The hwmon core
> serializes its own callers, but the debugfs files call
> corsairpsu_get_value() directly and never take that lock, so a debugfs
> read can land between another reader's rail select and its value read.
> 
> The result is a value from the wrong rail reported as the right one,
> because corsairpsu_usb_cmd() only checks the command echo and both
> transfers echo the command it expects. It can also make a caller consume
> the reply meant for the other one, since raw_event() writes into the
> shared buffer and completes whoever happens to be waiting.
> 
> Locking was dropped in commit 4207069edbf0 ("hwmon: (corsair-psu) Rely
> on subsystem locking") on the grounds that the subsystem serializes for
> us, which holds for sysfs but not for these files. Take
> the same lock in the debugfs paths that issue commands, using the guard
> added in commit d1e720c7328e ("hwmon: Support guard() and scoped_guard
> for subsystem locks"), as suggested in [1].
> 
> The lock cannot go into corsairpsu_request() itself: the hwmon core
> already holds it across ->read, so every sysfs read would deadlock.
> vendor_show() and product_show() only print strings cached during probe
> and issue no command, and corsairpsu_get_criticals() and
> corsairpsu_check_cmd_support() run before either interface is
> registered, so none of them need it.
> 
> [1] https://lore.kernel.org/all/5f0406fa-9692-49f0-bcfe-c013f5fc7b62@roeck-us.net/
> 
> Fixes: 4207069edbf0 ("hwmon: (corsair-psu) Rely on subsystem locking")
> Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
> ---
> This is the fix Guenter asked for in the May thread, written the way he
> suggested there. Wilken's patch used a driver private mutex around
> corsairpsu_request(); that thread stalled and the race is still present.
> 
> Wilken, does this cover the chained command case you were worried about?
> As far as I can tell it does: the whole select-rail plus read sequence
> now runs under the same lock the hwmon core takes around ->read, so a
> debugfs reader cannot land in the middle of one. If you had a case in
> mind that this misses, I would rather hear it than guess.

Yes, I think that is what Guenter asked me to test. There is actually a
way to get all values from the PSU at once. You can chain together all
the commands and everything supported should even fit into a single USB
HID frame (64bytes). That would make everything a bit easier. Though,
sorry that I did not go on with that. About a day after this someone put
basically all my open source projects through an AI agent and since then
I get bombarded with AI slop. I currently have not much energy (and fun)
left doing my projects.

I think I will test it in the next days.

greetings Wilken

> I have no Corsair PSU, so this is reasoned from the code rather than
> measured on hardware. What I did check:
> 
>   - hwmon_lock() takes hwdev->lock, and the hwmon core takes the same
>     mutex around ->read and ->write, so this really does serialize the
>     two entry points
>   - the lock therefore cannot go into corsairpsu_request(), the sysfs
>     path would deadlock on itself
>   - probe registers the hwmon device before creating the debugfs files
>     and remove tears them down in the opposite order, so priv->hwmon_dev
>     is always valid inside a debugfs read
> 
> Prior discussion:
>   https://lore.kernel.org/all/agR9YW7hGTJ_l7ms@monster.localdomain/
> 
>  drivers/hwmon/corsair-psu.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/hwmon/corsair-psu.c b/drivers/hwmon/corsair-psu.c
> index ce958cdaef58..24100519cd83 100644
> --- a/drivers/hwmon/corsair-psu.c
> +++ b/drivers/hwmon/corsair-psu.c
> @@ -664,6 +664,8 @@ static void print_uptime(struct seq_file *seqf, u8 cmd)
>  	long val;
>  	int ret;
>  
> +	guard(hwmon_lock)(priv->hwmon_dev);
> +
>  	ret = corsairpsu_get_value(priv, cmd, 0, &val);
>  	if (ret < 0) {
>  		seq_puts(seqf, "N/A\n");
> @@ -730,6 +732,8 @@ static int ocpmode_show(struct seq_file *seqf, void *unused)
>  	 * getting of the value itself can also fail during this. Because of this every other
> value
>  	 * than OCP_MULTI_RAIL can be considered as "single rail".
>  	 */
> +	guard(hwmon_lock)(priv->hwmon_dev);
> +
>  	ret = corsairpsu_get_value(priv, PSU_CMD_OCPMODE, 0, &val);
>  	if (ret < 0)
>  		seq_puts(seqf, "N/A\n");
> 
> base-commit: 2d2338c93da79b3bfe4b6099a931d9468d539952