fix(security): Add VLLM_MAX_N_SEQUENCES environment variable and enforce limit (#37952)

Signed-off-by: jperezde <jperezde@redhat.com>
Signed-off-by: Russell Bryant <rbryant@redhat.com>
Co-authored-by: Russell Bryant <rbryant@redhat.com>

tests/entrypoints/openai/chat_completion/test_chat.py

@@ -1020,3 +1020,114 @@ def test_chat_completion_request_n_parameter_various_values():
         assert sampling_params.n == n_value, (
             f"Expected n={n_value}, got n={sampling_params.n}"
         )
+
+
+def test_chat_completion_request_n_parameter_exceeds_default_limit(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """Test that n values exceeding the default limit are rejected."""
+    import vllm.envs as envs
+
+    monkeypatch.delenv("VLLM_MAX_N_SEQUENCES", raising=False)
+    if hasattr(envs.__getattr__, "cache_clear"):
+        envs.__getattr__.cache_clear()
+
+    max_n = envs.VLLM_MAX_N_SEQUENCES
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Test"}],
+        n=max_n + 1,
+        max_tokens=10,
+    )
+
+    with pytest.raises(ValueError, match="n must be at most"):
+        request.to_sampling_params(
+            max_tokens=10,
+            default_sampling_params={},
+        )
+
+
+def test_chat_completion_request_n_parameter_at_limit(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """Test that n at exactly the limit is accepted."""
+    import vllm.envs as envs
+
+    monkeypatch.delenv("VLLM_MAX_N_SEQUENCES", raising=False)
+    if hasattr(envs.__getattr__, "cache_clear"):
+        envs.__getattr__.cache_clear()
+
+    max_n = envs.VLLM_MAX_N_SEQUENCES
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Test"}],
+        n=max_n,
+        max_tokens=10,
+    )
+
+    sampling_params = request.to_sampling_params(
+        max_tokens=10,
+        default_sampling_params={},
+    )
+    assert sampling_params.n == max_n
+
+
+def test_chat_completion_request_n_parameter_custom_limit(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """Test that VLLM_MAX_N_SEQUENCES env var overrides the default limit."""
+    import vllm.envs as envs
+
+    monkeypatch.setenv("VLLM_MAX_N_SEQUENCES", "128")
+    if hasattr(envs.__getattr__, "cache_clear"):
+        envs.__getattr__.cache_clear()
+
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Test"}],
+        n=128,
+        max_tokens=10,
+    )
+
+    sampling_params = request.to_sampling_params(
+        max_tokens=10,
+        default_sampling_params={},
+    )
+    assert sampling_params.n == 128
+
+    request_over = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Test"}],
+        n=129,
+        max_tokens=10,
+    )
+
+    with pytest.raises(ValueError, match="n must be at most 128"):
+        request_over.to_sampling_params(
+            max_tokens=10,
+            default_sampling_params={},
+        )
+
+
+def test_chat_completion_request_n_parameter_massive_value(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """Test that astronomically large n values are rejected (CVE fix)."""
+    import vllm.envs as envs
+
+    monkeypatch.delenv("VLLM_MAX_N_SEQUENCES", raising=False)
+    if hasattr(envs.__getattr__, "cache_clear"):
+        envs.__getattr__.cache_clear()
+
+    request = ChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "Test"}],
+        n=100_000_000,
+        max_tokens=1,
+    )
+
+    with pytest.raises(ValueError, match="n must be at most"):
+        request.to_sampling_params(
+            max_tokens=1,
+            default_sampling_params={},
+        )