[PATCH] usb: typec: ucsi: ccg: Validate altmode index in GET_CURRENT_CAM response

[pip-izony <eeodqql09@gmail.com>]

From: Seungjin Bae <eeodqql09@gmail.com>

When the PPM reports more than one DisplayPort alternate mode for a
connector, ucsi_ccg_update_altmodes() merges them into a single entry
in uc->updated[] and sets uc->has_multiple_dp. In that case,
ucsi_ccg_update_get_current_cam_cmd() rewrites the response of the
GET_CURRENT_CAM command. The response is a single byte holding the
index of the currently active alternate mode, and it is provided by
the PPM firmware.

The function uses this byte directly as an index into uc->orig[], and
then uses the linked_idx read from that entry as the translated index
into uc->updated[]. Both arrays have UCSI_MAX_ALTMODES entries, but
neither index is checked against that size.

If a malicious or buggy PPM reports a value of UCSI_MAX_ALTMODES or
larger, e.g. 0xFF, uc->orig[cam].linked_idx reads over the end of
uc->orig[]. The byte read from there is then used as the index for
writing cam into uc->updated[new_cam].active_idx, so the out-of-bounds
read is followed by an out-of-bounds write. This happens without any
userspace action, since the UCSI core issues GET_CURRENT_CAM on its own
when handling connector changes.

Fix this by ignoring responses whose index is out of range and leaving
the original value in place. The UCSI core only uses the value as an
index into con->port_altmode[] when it is below UCSI_MAX_ALTMODES, and
otherwise treats it as no active alternate mode. Also check linked_idx
before using it as an index, so that the write into uc->updated[] is
always within bounds.

Fixes: 170a6726d0e2 ("usb: typec: ucsi: add support for separate DP altmode devices")
Cc: stable@vger.kernel.org
Reported-by: Nathan Rebello <nathan.c.rebello.27@dartmouth.edu>
Signed-off-by: Seungjin Bae <eeodqql09@gmail.com>
---
The issue was found through code audit and was reported privately,
so there is no public report to link to.

 drivers/usb/typec/ucsi/ucsi_ccg.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/usb/typec/ucsi/ucsi_ccg.c b/drivers/usb/typec/ucsi/ucsi_ccg.c
index 91c2958a708c..4d1166c20639 100644
--- a/drivers/usb/typec/ucsi/ucsi_ccg.c
+++ b/drivers/usb/typec/ucsi/ucsi_ccg.c
@@ -389,7 +389,13 @@ static void ucsi_ccg_update_get_current_cam_cmd(struct ucsi_ccg *uc, u8 *data)
 	u8 cam, new_cam;
 
 	cam = data[0];
+	if (cam >= UCSI_MAX_ALTMODES)
+		return;
+
 	new_cam = uc->orig[cam].linked_idx;
+	if (new_cam >= UCSI_MAX_ALTMODES)
+		return;
+
 	uc->updated[new_cam].active_idx = cam;
 	data[0] = new_cam;
 }
-- 
2.43.0