[PATCH] drm/amd/display: Fix a NULL pointer dereference in amdgpu_dm_connector_add_common_modes()

[Zhou Qingyang <zhou1615@umn.edu>]

In amdgpu_dm_connector_add_common_modes(), amdgpu_dm_create_common_mode()
is assigned to mode and is passed to drm_mode_probed_add() directly after
that. drm_mode_probed_add() passes &mode->head to list_add_tail(), and
there is a dereference of it in list_add_tail() without recoveries, which
could lead to NULL pointer dereference on failure of
amdgpu_dm_create_common_mode().

Fix this by adding a NULL check of mode.

This bug was found by a static analyzer.

Builds with 'make allyesconfig' show no new warnings,
and our static analyzer no longer warns about this code.

Fixes: e7b07ceef2a6 ("drm/amd/display: Merge amdgpu_dm_types and amdgpu_dm")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
---
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.

 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 7f9773f8dab6..9ad94186b146 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -8143,6 +8143,9 @@ static void amdgpu_dm_connector_add_common_modes(struct drm_encoder *encoder,
 		mode = amdgpu_dm_create_common_mode(encoder,
 				common_modes[i].name, common_modes[i].w,
 				common_modes[i].h);
+		if (!mode)
+			continue;
+
 		drm_mode_probed_add(connector, mode);
 		amdgpu_dm_connector->num_modes++;
 	}
-- 
2.25.1