Re: [PATCH] platform/chrome: cros_ec_proto: Lock device when updating MKBP version

[Tzung-Bi Shih <tzungbi@kernel.org>]

On Mon, Jul 29, 2024 at 01:57:09PM +0200, Patryk Duda wrote:
> On Mon, Jul 29, 2024 at 5:47 AM Tzung-Bi Shih <tzungbi@kernel.org> wrote:
> >
> > On Thu, Jul 25, 2024 at 05:57:13PM +0000, Patryk Duda wrote:
> > > The cros_ec_get_host_command_version_mask() function requires that the
> > > caller must have ec_dev->lock mutex before calling it. This requirement
> > > was not met and as a result it was possible that two commands were sent
> > > to the device at the same time.
> >
> > To clarify:
> > - What would happen if multiple cros_ec_get_host_command_version_mask() calls
> >   at the same time?
> In the best case, MCU will receive both commands glued together and
> will ignore them.
> It will result in a timeout in the kernel. In the worst case, request
> and/or response buffers will be
> corrupted.
> 
> > - What are the callees?  I'm trying to understand the source of parallelism.
> This is a race between interrupt handling and ioctl call from userspace
> 
> Handling interrupt path
> cros_ec_irq_thread()
> cros_ec_handle_event()
> cros_ec_get_next_event() - Queries host command version without taking
> ec_dev->lock mutex first
> cros_ec_get_host_command_version_mask()
> cros_ec_send_command()
> cros_ec_xfer_command()
> cros_ec_uart_pkt_xfer()
> 
> Command from userspace
> cros_ec_chardev_ioctl()
> cros_ec_chardev_ioctl_xcmd()
> cros_ec_cmd_xfer() - Locks ec_dev->lock mutex before sending command
> cros_ec_send_command()
> cros_ec_xfer_command()
> cros_ec_uart_pkt_xfer()
> 
> >
> > Also, the patch also needs an unlock at [1].
> >
> > [1]: https://elixir.bootlin.com/linux/v6.10/source/drivers/platform/chrome/cros_ec_proto.c#L819
> 
> Yeah. I'll fix it in v2

I'm wondering if it's simpler to just lock and unlock around calling
cros_ec_get_host_command_version_mask().  What do you think?

> > > The problem was observed while using UART backend which doesn't use any
> > > additional locks, unlike SPI backend which locks the controller until
> > > response is received.
> >
> > Is it a general issue if multiple commands send to EC at a time?  If yes, it
> > should serialize that in the UART transportation.
> 
> Host Commands only support one command at a time. It's enforced by 'lock' mutex
> from cros_ec_device structure. We just need to use it properly.

I see.  Please use the fixes tag if you'd have chance to send next version:
Fixes: f74c7557ed0d ("platform/chrome: cros_ec_proto: Update version on GET_NEXT_EVENT failure")