[PATCH 6.6.y] drm/amd/display: Ensure array index tg_inst won't be -1

[Artem Dinaburg]

From: Alex Hung <alex.hung@xxxxxxx>

[ Upstream commit 687fe329f18ab0ab0496b20ed2cb003d4879d931 ]

[WHY & HOW]
tg_inst will be a negative if timing_generator_count equals 0, which
should be checked before used.

This fixes 2 OVERRUN issues reported by Coverity.

[ Backport to 6.6.y: mapped the newer pipe index and helper to i in the
  older acquire_first_free_pipe(). ]

Reviewed-by: Harry Wentland <harry.wentland@xxxxxxx>
Acked-by: Tom Chung <chiahsuan.chung@xxxxxxx>
Signed-off-by: Alex Hung <alex.hung@xxxxxxx>
Tested-by: Daniel Wheeler <daniel.wheeler@xxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@xxxxxxxxxxxxxxx>
---
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-2024-46730. It adds upstream's zero-count guard
before the fallback timing-generator index is computed.

The condition now avoids forming timing_generators[-1] if a zero-count pool
reaches this helper. I did not find a successfully constructed in-tree
resource pool with zero timing generators on this revision, so this is a
defensive match to the upstream check rather than a reproduced fault.

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-2024-46730
Upstream: 687fe329f18ab0ab0496b20ed2cb003d4879d931

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_resource.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_resource.c b/drivers/gpu/drm/amd/display/dc/core/dc_resource.c
index 84474d5122284a..1f576b2a1960fd 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_resource.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_resource.c
@@ -2354,7 +2354,8 @@ static int acquire_first_free_pipe(
 				pipe_ctx->plane_res.mpcc_inst = pool->dpps[i]->inst;
 			pipe_ctx->pipe_idx = i;
 
-			if (i >= pool->timing_generator_count) {
+			if (i >= pool->timing_generator_count &&
+			    pool->timing_generator_count != 0) {
 				int tg_inst = pool->timing_generator_count - 1;
 
 				pipe_ctx->stream_res.tg = pool->timing_generators[tg_inst];
-- 
2.39.5