[PATCH] drm: bridge: cdns-mhdp8546: Fix a NULL pointer dereference in cdns_mhdp_atomic_enable()

[Zhou Qingyang]

In cdns_mhdp_atomic_enable(), the return value of drm_mode_duplicate()
is assigned to mhdp_state->current_mode and used in drm_mode_set_name().
There is a dereference of it in drm_mode_set_name(), which could lead
to a NULL pointer dereference on failure of drm_mode_duplicate().

Fix this bug by adding a check of mhdp_state->current_mode.

This bug was found by a static analyzer. The analysis employs
differential checking to identify inconsistent security operations
(e.g., checks or kfrees) between two code paths and confirms that the
inconsistent operations are not recovered in the current function or
the callers, so they constitute bugs.

Note that, as a bug found by static analysis, it can be a false
positive or hard to trigger. Multiple researchers have cross-reviewed
the bug.

Builds with CONFIG_DRM_CDNS_MHDP8546=m show no new warnings,
and our static analyzer no longer warns about this code.

Fixes: fb43aa0acdfd ("drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge")
Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
---
 drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index 5530fbf64f1e..347fbecf76a4 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
@@ -2040,6 +2040,11 @@ static void cdns_mhdp_atomic_enable(struct drm_bridge *bridge,
 	mhdp_state = to_cdns_mhdp_bridge_state(new_state);
 
 	mhdp_state->current_mode = drm_mode_duplicate(bridge->dev, mode);
+	if (!mhdp_state->current_mode) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	drm_mode_set_name(mhdp_state->current_mode);
 
 	dev_dbg(mhdp->dev, "%s: Enabling mode %s\n", __func__, mode->name);
-- 
2.25.1