[PATCH 6.6.y] drm/amd/display: Add a dc_state NULL check in dc_state_release

[Artem Dinaburg <artem@trailofbits.com>]

From: Allen Pan <allen.pan@amd.com>

[ Upstream commit 334b56cea5d9df5989be6cf1a5898114fa70ad98 ]

[How]
Check wheather state is NULL before releasing it.

[ Backport to 6.6.y: implemented the same guard in older
  dc_release_state() in dc.c rather than dc_state_release() in
  dc_state.c. ]

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Allen Pan <allen.pan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
Hi Greg, Sasha, and drm amd maintainers,

I am working through the small CVE backports still missing from 6.6.y.
This one addresses CVE-2024-26948. It adds upstream's NULL guard to the
older display-state release helper.

The helper is used by display-state cleanup paths and currently
dereferences its argument unconditionally. The patch makes the helper
NULL-safe.

The fix is already present in 6.12.y, 6.18.y, and 7.2.y, but not in 6.6.y.
This fix also affects 6.1.y, which will need a separate backport; this
submission contains only the 6.6.y patch.
The target-specific adjustment is recorded in the bracketed note above.

Could you please queue it for 6.6.y?

CVE: CVE-2024-26948
Upstream: 334b56cea5d9df5989be6cf1a5898114fa70ad98

AI assistance: An LLM helped identify, adapt, and validate this backport; I
reviewed the resulting code and validation evidence.

Thanks,
Artem Dinaburg

 drivers/gpu/drm/amd/display/dc/core/dc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc.c b/drivers/gpu/drm/amd/display/dc/core/dc.c
index bea18f450ac9c7..6ba67a20eb4846 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc.c
@@ -2302,7 +2302,8 @@ static void dc_state_free(struct kref *kref)
 
 void dc_release_state(struct dc_state *context)
 {
-	kref_put(&context->refcount, dc_state_free);
+	if (context != NULL)
+		kref_put(&context->refcount, dc_state_free);
 }
 
 bool dc_set_generic_gpio_for_stereo(bool enable,
-- 
2.39.5