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: sta...@vger.kernel.org # 6.3
> Cc: Bjorn Andersson <anders...@kernel.org>
> Cc: Dmitry Baryshkov <dmitry.barysh...@linaro.org>
> Signed-off-by: Johan Hovold <johan+lin...@kernel.org>
Reviewed-by: Bjorn Andersson <anders...@kernel.org>
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
>
