drm/amdgpu: disallow multiple FENCE chunks in one submit

amdgpu_cs_pass1() dispatches on chunk_id once per chunk without
rejecting repeated ids. p->uf_bo is a single-slot field, so a
submission carrying two AMDGPU_CHUNK_ID_FENCE chunks runs
amdgpu_cs_p1_user_fence() twice, and the second run overwrites
p->uf_bo with a freshly referenced BO without dropping the reference
taken by the first.

amdgpu_cs_parser_fini() only unrefs the final p->uf_bo, so every FENCE
chunk but the last leaks a BO reference. The leaked BO outlives handle
close and process exit.

Reject duplicate FENCE chunks the same way commit fec5f8e8c6
("drm/amdgpu: disallow multiple BO_HANDLES chunks in one submit") did
for p->bo_list.

Fixes: d38ceaf99e ("drm/amdgpu: add core driver (v4)")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Assisted-by: Claude:claude-opus-5
Cc: stable@vger.kernel.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 665b1fc2a1)

drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c

@@ -248,6 +248,10 @@ static int amdgpu_cs_pass1(struct amdgpu_cs_parser *p,
 			if (size < sizeof(struct drm_amdgpu_cs_chunk_fence))
 				goto free_partial_kdata;
 
+			/* Only a single user fence is allowed to simplify handling. */
+			if (p->uf_bo)
+				goto free_partial_kdata;
+
 			ret = amdgpu_cs_p1_user_fence(p, p->chunks[i].kdata,
 						      &uf_offset);
 			if (ret)