Re: [PATCH 3/6] soc: qcom: pmic_glink_altmode: fix drm bridge use-after-free

[Bjorn Andersson]

On Sat, Feb 17, 2024 at 04:02:25PM +0100, Johan Hovold wrote:
> A recent DRM series purporting to simplify support for "transparent
> bridges" and handling of probe deferrals ironically exposed a
> use-after-free issue on pmic_glink_altmode probe deferral.
> 
> This has manifested itself as the display subsystem occasionally failing
> to initialise and NULL-pointer dereferences during boot of machines like
> the Lenovo ThinkPad X13s.
> 
> Specifically, the dp-hpd bridge is currently registered before all
> resources have been acquired which means that it can also be
> deregistered on probe deferrals.
> 
> In the meantime there is a race window where the new aux bridge driver
> (or PHY driver previously) may have looked up the dp-hpd bridge and
> stored a (non-reference-counted) pointer to the bridge which is about to
> be deallocated.
> 
> When the display controller is later initialised, this triggers a
> use-after-free when attaching the bridges:
> 
> 	dp -> aux -> dp-hpd (freed)
> 
> which may, for example, result in the freed bridge failing to attach:
> 
> 	[drm:drm_bridge_attach [drm]] *ERROR* failed to attach bridge /soc@0/phy@88eb000 to encoder TMDS-31: -16
> 
> or a NULL-pointer dereference:
> 
> 	Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> 	...
> 	Call trace:
> 	  drm_bridge_attach+0x70/0x1a8 [drm]
> 	  drm_aux_bridge_attach+0x24/0x38 [aux_bridge]
> 	  drm_bridge_attach+0x80/0x1a8 [drm]
> 	  dp_bridge_init+0xa8/0x15c [msm]
> 	  msm_dp_modeset_init+0x28/0xc4 [msm]
> 
> The DRM bridge implementation is clearly fragile and implicitly built on
> the assumption that bridges may never go away. In this case, the fix is
> to move the bridge registration in the pmic_glink_altmode driver to
> after all resources have been looked up.
> 
> Incidentally, with the new dp-hpd bridge implementation, which registers
> child devices, this is also a requirement due to a long-standing issue
> in driver core that can otherwise lead to a probe deferral loop (see
> fbc35b45f9f6 ("Add documentation on meaning of -EPROBE_DEFER")).
> 
> Fixes: 080b4e24852b ("soc: qcom: pmic_glink: Introduce altmode support")
> Fixes: 2bcca96abfbf ("soc: qcom: pmic-glink: switch to DRM_AUX_HPD_BRIDGE")
> Cc: stable@xxxxxxxxxxxxxxx      # 6.3
> Cc: Bjorn Andersson <andersson@xxxxxxxxxx>
> Cc: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>

Reviewed-by: Bjorn Andersson <andersson@xxxxxxxxxx>

Regards,
Bjorn

> ---
>  drivers/soc/qcom/pmic_glink_altmode.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/soc/qcom/pmic_glink_altmode.c b/drivers/soc/qcom/pmic_glink_altmode.c
> index 5fcd0fdd2faa..b3808fc24c69 100644
> --- a/drivers/soc/qcom/pmic_glink_altmode.c
> +++ b/drivers/soc/qcom/pmic_glink_altmode.c
> @@ -76,7 +76,7 @@ struct pmic_glink_altmode_port {
>  
>  	struct work_struct work;
>  
> -	struct device *bridge;
> +	struct auxiliary_device *bridge;
>  
>  	enum typec_orientation orientation;
>  	u16 svid;
> @@ -230,7 +230,7 @@ static void pmic_glink_altmode_worker(struct work_struct *work)
>  	else
>  		pmic_glink_altmode_enable_usb(altmode, alt_port);
>  
> -	drm_aux_hpd_bridge_notify(alt_port->bridge,
> +	drm_aux_hpd_bridge_notify(&alt_port->bridge->dev,
>  				  alt_port->hpd_state ?
>  				  connector_status_connected :
>  				  connector_status_disconnected);
> @@ -454,7 +454,7 @@ static int pmic_glink_altmode_probe(struct auxiliary_device *adev,
>  		alt_port->index = port;
>  		INIT_WORK(&alt_port->work, pmic_glink_altmode_worker);
>  
> -		alt_port->bridge = drm_dp_hpd_bridge_register(dev, to_of_node(fwnode));
> +		alt_port->bridge = devm_drm_dp_hpd_bridge_alloc(dev, to_of_node(fwnode));
>  		if (IS_ERR(alt_port->bridge)) {
>  			fwnode_handle_put(fwnode);
>  			return PTR_ERR(alt_port->bridge);
> @@ -510,6 +510,16 @@ static int pmic_glink_altmode_probe(struct auxiliary_device *adev,
>  		}
>  	}
>  
> +	for (port = 0; port < ARRAY_SIZE(altmode->ports); port++) {
> +		alt_port = &altmode->ports[port];
> +		if (!alt_port->bridge)
> +			continue;
> +
> +		ret = devm_drm_dp_hpd_bridge_add(dev, alt_port->bridge);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	altmode->client = devm_pmic_glink_register_client(dev,
>  							  altmode->owner_id,
>  							  pmic_glink_altmode_callback,
> -- 
> 2.43.0
>