[PATCH v4] usb: typec: bus: verify partner exists in typec_altmode_attention

[RD Babiera]

Some usb hubs will negotiate DisplayPort Alt mode with the device
but will then negotiate a data role swap after entering the alt
mode. The data role swap causes the device to unregister all alt
modes, however the usb hub will still send Attention messages
even after failing to reregister the Alt Mode. type_altmode_attention
currently does not verify whether or not a device's altmode partner
exists, which results in a NULL pointer error when dereferencing
the typec_altmode and typec_altmode_ops belonging to the altmode
partner.

Verify the presence of a device's altmode partner before sending
the Attention message to the Alt Mode driver.

Fixes: 8a37d87d72f0 ("usb: typec: Bus type for alternate modes")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: RD Babiera <rdbabiera@xxxxxxxxxx>
---
Changes since v1:
* Only assigns pdev if altmode partner exists in typec_altmode_attention
* Removed error return in typec_altmode_attention if Alt Mode does
  not implement Attention messages.
* Changed tcpm_log message to indicate that altmode partner does not exist,
  as it only logs in that case.
---
Changes since v2:
* Changed tcpm_log message to accurately reflect error
* Revised commit message
---
Changes since v3:
* Fixed nits
---
 drivers/usb/typec/bus.c           | 12 ++++++++++--
 drivers/usb/typec/tcpm/tcpm.c     |  3 ++-
 include/linux/usb/typec_altmode.h |  2 +-
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/typec/bus.c b/drivers/usb/typec/bus.c
index fe5b9a2e61f5..e95ec7e382bb 100644
--- a/drivers/usb/typec/bus.c
+++ b/drivers/usb/typec/bus.c
@@ -183,12 +183,20 @@ EXPORT_SYMBOL_GPL(typec_altmode_exit);
  *
  * Notifies the partner of @adev about Attention command.
  */
-void typec_altmode_attention(struct typec_altmode *adev, u32 vdo)
+int typec_altmode_attention(struct typec_altmode *adev, u32 vdo)
 {
-	struct typec_altmode *pdev = &to_altmode(adev)->partner->adev;
+	struct altmode *partner = to_altmode(adev)->partner;
+	struct typec_altmode *pdev;
+
+	if (!partner)
+		return -ENODEV;
+
+	pdev = &partner->adev;
 
 	if (pdev->ops && pdev->ops->attention)
 		pdev->ops->attention(pdev, vdo);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(typec_altmode_attention);
 
diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
index 5a7d8cc04628..77fe16190766 100644
--- a/drivers/usb/typec/tcpm/tcpm.c
+++ b/drivers/usb/typec/tcpm/tcpm.c
@@ -1877,7 +1877,8 @@ static void tcpm_handle_vdm_request(struct tcpm_port *port,
 			}
 			break;
 		case ADEV_ATTENTION:
-			typec_altmode_attention(adev, p[1]);
+			if (typec_altmode_attention(adev, p[1]))
+				tcpm_log(port, "typec_altmode_attention no port partner altmode");
 			break;
 		}
 	}
diff --git a/include/linux/usb/typec_altmode.h b/include/linux/usb/typec_altmode.h
index 350d49012659..28aeef8f9e7b 100644
--- a/include/linux/usb/typec_altmode.h
+++ b/include/linux/usb/typec_altmode.h
@@ -67,7 +67,7 @@ struct typec_altmode_ops {
 
 int typec_altmode_enter(struct typec_altmode *altmode, u32 *vdo);
 int typec_altmode_exit(struct typec_altmode *altmode);
-void typec_altmode_attention(struct typec_altmode *altmode, u32 vdo);
+int typec_altmode_attention(struct typec_altmode *altmode, u32 vdo);
 int typec_altmode_vdm(struct typec_altmode *altmode,
 		      const u32 header, const u32 *vdo, int count);
 int typec_altmode_notify(struct typec_altmode *altmode, unsigned long conf,

base-commit: f176638af476c6d46257cc3303f5c7cf47d5967d
-- 
2.41.0.694.ge786442a9b-goog