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fix(mqtt): pin the CA accept path, honour presented intermediates, classify the dispose race

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Review follow-ups on MqttConnection (Task 3):

- Every certificate test asserted rejection, so the accept branch was
  unreachable-by-regression. Adds a leaf genuinely issued by the pinned CA and
  asserts acceptance.
- ValidateAgainstPinnedCa never seeded ChainPolicy.ExtraStore from the incoming
  chain, so a leaf behind an intermediate delivered during the handshake failed
  despite a legitimate path to the pinned root. Seeds from both the incoming
  chain's elements and its ExtraStore; CustomRootTrust still means only the
  pinned roots may terminate the chain.
- A DisposeAsync racing an in-flight connect escaped as an unclassified
  exception; it now folds into ObjectDisposedException.
- Promotes the single-caller concurrency invariant into the type remarks, with
  the accurate blast radius (a leaked live connection, not a benign throw).
  Serialising the lifecycle remains Task 4's job.
- X509Chain.Build can throw; an exception escaping a TLS validation callback is
  an opaque handshake crash, so it is caught and refused.
- Adds a connect-retry test (Task 4's reconnect loop reuses the instance) and a
  disposed-then-connect test.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
This commit is contained in:
Joseph Doherty
2026-07-24 14:38:16 -04:00
parent ded4c41798
commit 5629f3698d
2 changed files with 239 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttConnection.cs
+69 -3
View File
@@ -1,4 +1,5 @@
using System.Net.Security;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using Microsoft.Extensions.Logging;
using MQTTnet;
@@ -29,6 +30,18 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Mqtt;
/// <see cref="MqttDriverOptions.Password"/>; the options record itself redacts it in
/// <c>ToString()</c>.
/// </para>
/// <para>
/// <b>NOT thread-safe — single-caller by contract.</b> <see cref="ConnectAsync"/> and
/// <see cref="DisposeAsync"/> take no lock and must never run concurrently. Calling them
/// concurrently <b>leaks a live broker connection</b>: <see cref="DisposeAsync"/> can
/// observe <c>_disposed == false</c>, set it, exchange a still-null <c>_client</c> for
/// null, and return having disposed nothing — after which the in-flight
/// <see cref="ConnectAsync"/> creates a client, assigns it and connects successfully.
/// That client holds an open socket that no later <see cref="DisposeAsync"/> can ever
/// reach, because <c>_disposed</c> is already <c>true</c>. Serialising the lifecycle is
/// the job of the reconnect supervisor added in Task 4; it is deliberately not solved
/// here.
/// </para>
/// Reconnect/backoff (Task 4) and subscription (Task 6) are deliberately not implemented here.
/// </remarks>
public sealed class MqttConnection : IAsyncDisposable
@@ -96,16 +109,24 @@ public sealed class MqttConnection : IAsyncDisposable
/// Connects to the broker, failing at <see cref="MqttDriverOptions.ConnectTimeoutSeconds"/>
/// rather than waiting indefinitely on an unresponsive peer.
/// </summary>
/// <remarks>
/// May be retried on the same instance after a failed attempt — the underlying
/// <see cref="IMqttClient"/> is created once and reused across attempts, which is what the
/// Task-4 reconnect loop depends on. Not safe to call concurrently with itself or with
/// <see cref="DisposeAsync"/>; see the type-level remarks.
/// </remarks>
/// <param name="cancellationToken">Caller cancellation; linked with the connect deadline.</param>
/// <exception cref="TimeoutException">The connect deadline elapsed.</exception>
/// <exception cref="OperationCanceledException"><paramref name="cancellationToken"/> was cancelled.</exception>
/// <exception cref="ObjectDisposedException">
/// The connection was disposed, either before the attempt started or while it was in flight.
/// </exception>
public async Task ConnectAsync(CancellationToken cancellationToken)
{
ObjectDisposedException.ThrowIf(_disposed, this);
// Single-caller by contract: the reconnect supervisor (Task 4) owns connect/disconnect
// sequencing, so no lock is taken here. The options are rebuilt per attempt so a rotated
// CA file is picked up on the next connect rather than being pinned for the process life.
// The options are rebuilt per attempt so a rotated CA file is picked up on the next
// connect rather than being pinned for the life of the process.
var client = _client ??= new MqttClientFactory().CreateMqttClient();
var clientOptions = BuildClientOptions(_options, clientIdSuffix: null, _logger);
@@ -124,6 +145,18 @@ public sealed class MqttConnection : IAsyncDisposable
{
await client.ConnectAsync(clientOptions, linked.Token).ConfigureAwait(false);
}
// A concurrent DisposeAsync disposes the client out from under the awaiting connect, and
// whatever MQTTnet throws for that matches neither token filter. Classify it rather than
// letting an undocumented exception escape the contract below.
catch (Exception ex) when (_disposed)
{
throw new ObjectDisposedException(
nameof(MqttConnection),
new InvalidOperationException(
$"MQTT driver '{_driverId}': connect to {_options.Host}:{_options.Port} was aborted because the "
+ "connection was disposed while the attempt was in flight.",
ex));
}
// MQTTnet wraps a cancelled connect in MqttConnectingFailedException rather than letting
// the OperationCanceledException surface, so the two legs are told apart by which token
// fired, not by the exception type. Caller intent wins over the deadline.
@@ -297,8 +330,30 @@ public sealed class MqttConnection : IAsyncDisposable
using var chain = new X509Chain();
chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;
chain.ChainPolicy.CustomTrustStore.AddRange(trustedRoots);
// Revocation is deliberately not checked: the pin exists precisely for self-issued
// dev / on-prem CAs, which typically publish no CRL or OCSP responder, so checking
// would fail every such chain. The pin itself is the revocation story — remove the CA.
chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
// Seed the intermediates the broker presented during the handshake. A leaf signed by
// an intermediate that chains up to the pinned root is a common broker topology, and
// that intermediate arrives on the wire rather than being installed locally — without
// it in the candidate pool a legitimate trust path simply cannot be built.
// CustomRootTrust still means only `trustedRoots` may terminate the chain, so a rogue
// self-signed cert smuggled in here cannot become an anchor.
// Both sources are read: platforms differ in whether the peer-supplied intermediates
// land in the incoming chain's elements, its ExtraStore, or both.
if (args.Chain is not null)
{
foreach (var element in args.Chain.ChainElements)
{
chain.ChainPolicy.ExtraStore.Add(element.Certificate);
}
chain.ChainPolicy.ExtraStore.AddRange(args.Chain.ChainPolicy.ExtraStore);
}
if (chain.Build(presented))
{
return true;
@@ -310,6 +365,17 @@ public sealed class MqttConnection : IAsyncDisposable
string.Join(", ", chain.ChainStatus.Select(s => s.Status)));
return false;
}
catch (CryptographicException ex)
{
// X509Chain.Build throws on a malformed certificate or an unusable policy. An exception
// escaping a TLS validation callback surfaces as an opaque handshake crash, so classify
// it here and refuse: an unverifiable certificate is a rejected certificate.
logger?.LogError(
ex,
"MQTT broker certificate could not be validated against the pinned CA '{CaCertificatePath}'; rejecting.",
caCertificatePath);
return false;
}
finally
{
owned?.Dispose();
tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttConnectionTests.cs
+170 -18
View File
@@ -89,6 +89,45 @@ public sealed class MqttConnectionTests
await Should.ThrowAsync<TimeoutException>(async () => await conn.ConnectAsync(CancellationToken.None));
}
/// <summary>
/// Task 4's reconnect loop retries connect on the SAME instance, reusing the
/// <c>IMqttClient</c> held across failures. A failed attempt must therefore leave the
/// connection usable rather than poisoned.
/// </summary>
[Fact]
public async Task ConnectAsync_CanBeRetriedOnTheSameInstanceAfterAFailedAttempt()
{
using var blackhole = new BlackholeBroker();
var opts = new MqttDriverOptions
{
Host = "127.0.0.1", Port = blackhole.Port, UseTls = false, ConnectTimeoutSeconds = 1,
};
await using var conn = new MqttConnection(opts, driverId: "t", logger: null);
await Should.ThrowAsync<TimeoutException>(async () => await conn.ConnectAsync(CancellationToken.None));
// Second attempt reaches the broker again and fails the same way — not ObjectDisposed,
// not a null client, not a silently-swallowed no-op.
await Should.ThrowAsync<TimeoutException>(async () => await conn.ConnectAsync(CancellationToken.None));
conn.IsConnected.ShouldBeFalse();
}
/// <summary>
/// Connecting a disposed instance is an <see cref="ObjectDisposedException"/> — the same
/// classification the in-flight dispose race folds into. Dispose is idempotent.
/// </summary>
[Fact]
public async Task ConnectAsync_AfterDispose_ThrowsObjectDisposed_AndDisposeIsIdempotent()
{
var opts = new MqttDriverOptions { Host = "127.0.0.1", Port = 1, UseTls = false, ConnectTimeoutSeconds = 1 };
var conn = new MqttConnection(opts, driverId: "t", logger: null);
await conn.DisposeAsync();
await conn.DisposeAsync();
await Should.ThrowAsync<ObjectDisposedException>(async () => await conn.ConnectAsync(CancellationToken.None));
}
/// <summary>Never log or throw credentials — the plan's cross-cutting secrets rule.</summary>
[Fact]
public async Task ConnectAsync_Failure_DoesNotLeakPasswordIntoExceptionMessage()
@@ -153,21 +192,76 @@ public sealed class MqttConnectionTests
var tls = built.ChannelOptions.ShouldBeOfType<MqttClientTcpOptions>().TlsOptions;
tls.CertificateValidationHandler.ShouldNotBeNull();
tls.CertificateValidationHandler(SelfSignedValidationArgs(built)).ShouldBeFalse();
using var stranger = SelfSignedLeaf(Host);
tls.CertificateValidationHandler(ValidationArgs(built, stranger)).ShouldBeFalse();
}
/// <summary>An untrusted, unpinned server certificate is rejected — the pin actually validates.</summary>
[Fact]
public void BuildClientOptions_CaPin_RejectsCertificateNotIssuedByThePinnedCa()
{
var caPath = WriteTempPemCa();
using var ca = IssueCa(issuer: null, "otopcua-test-ca", validDays: 30);
var caPath = WritePem(ca);
try
{
var opts = new MqttDriverOptions { Host = Host, Port = 8883, UseTls = true, CaCertificatePath = caPath };
var built = MqttConnection.BuildClientOptions(opts, clientIdSuffix: null);
var tls = built.ChannelOptions.ShouldBeOfType<MqttClientTcpOptions>().TlsOptions;
tls.CertificateValidationHandler!(SelfSignedValidationArgs(built)).ShouldBeFalse();
using var stranger = SelfSignedLeaf(Host);
tls.CertificateValidationHandler!(ValidationArgs(built, stranger)).ShouldBeFalse();
}
finally
{
File.Delete(caPath);
}
}
/// <summary>
/// The accept path. Without this every certificate test asserts rejection, so deleting the
/// <c>return true</c> — or getting <c>TrustMode</c> / <c>CustomTrustStore</c> wrong — would
/// leave the suite green while no broker could ever connect.
/// </summary>
[Fact]
public void BuildClientOptions_CaPin_AcceptsLeafIssuedByThePinnedCa()
{
using var ca = IssueCa(issuer: null, "otopcua-test-ca", validDays: 30);
using var leaf = IssueLeaf(ca, Host, validDays: 20);
var caPath = WritePem(ca);
try
{
var opts = new MqttDriverOptions { Host = Host, Port = 8883, UseTls = true, CaCertificatePath = caPath };
var built = MqttConnection.BuildClientOptions(opts, clientIdSuffix: null);
var tls = built.ChannelOptions.ShouldBeOfType<MqttClientTcpOptions>().TlsOptions;
tls.CertificateValidationHandler!(ValidationArgs(built, leaf)).ShouldBeTrue();
}
finally
{
File.Delete(caPath);
}
}
/// <summary>
/// The common real-world broker topology: the leaf is signed by an intermediate that chains
/// to the pinned root, and the intermediate is delivered in the handshake rather than being
/// installed locally. Only the pinned ROOT is written to the CA file — the intermediate must
/// be picked up from what the broker presented.
/// </summary>
[Fact]
public void BuildClientOptions_CaPin_AcceptsLeafBehindAnIntermediateThatTheBrokerPresented()
{
using var root = IssueCa(issuer: null, "otopcua-test-root", validDays: 30);
using var intermediate = IssueCa(root, "otopcua-test-intermediate", validDays: 25);
using var leaf = IssueLeaf(intermediate, Host, validDays: 20);
var caPath = WritePem(root);
try
{
var opts = new MqttDriverOptions { Host = Host, Port = 8883, UseTls = true, CaCertificatePath = caPath };
var built = MqttConnection.BuildClientOptions(opts, clientIdSuffix: null);
var tls = built.ChannelOptions.ShouldBeOfType<MqttClientTcpOptions>().TlsOptions;
tls.CertificateValidationHandler!(ValidationArgs(built, leaf, intermediate)).ShouldBeTrue();
}
finally
{
@@ -290,29 +384,87 @@ public sealed class MqttConnectionTests
=> MqttConnection.BuildClientOptions(opts, clientIdSuffix: null)
.ChannelOptions.ShouldBeOfType<MqttClientTcpOptions>().TlsOptions;
private static MqttClientCertificateValidationEventArgs SelfSignedValidationArgs(MqttClientOptions built)
private static readonly DateTimeOffset NotBefore = DateTimeOffset.UtcNow.AddDays(-1);
/// <summary>A CA certificate, self-signed when <paramref name="issuer"/> is null.</summary>
private static X509Certificate2 IssueCa(X509Certificate2? issuer, string commonName, int validDays)
{
using var rsa = RSA.Create(2048);
var request = new CertificateRequest($"CN={Host}", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
var cert = request.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddDays(1));
return new MqttClientCertificateValidationEventArgs(
cert,
new X509Chain(),
SslPolicyErrors.RemoteCertificateChainErrors,
built.ChannelOptions);
using var key = RSA.Create(2048);
var request = new CertificateRequest($"CN={commonName}", key, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
request.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
request.CertificateExtensions.Add(
new X509KeyUsageExtension(X509KeyUsageFlags.KeyCertSign | X509KeyUsageFlags.CrlSign, true));
request.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(request.PublicKey, false));
if (issuer is null)
{
return request.CreateSelfSigned(NotBefore, NotBefore.AddDays(validDays));
}
private static string WriteTempPemCa()
// Create() drops the private key; an intermediate needs it back to sign its own children.
using var issued = request.Create(issuer, NotBefore, NotBefore.AddDays(validDays), NextSerial());
return issued.CopyWithPrivateKey(key);
}
/// <summary>A server (end-entity) certificate signed by <paramref name="issuer"/>.</summary>
private static X509Certificate2 IssueLeaf(X509Certificate2 issuer, string commonName, int validDays)
{
using var key = RSA.Create(2048);
var request = new CertificateRequest($"CN={commonName}", key, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
request.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, true));
request.CertificateExtensions.Add(
new X509KeyUsageExtension(X509KeyUsageFlags.DigitalSignature | X509KeyUsageFlags.KeyEncipherment, true));
request.CertificateExtensions.Add(
new X509EnhancedKeyUsageExtension([new Oid("1.3.6.1.5.5.7.3.1")], false)); // serverAuth
request.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(request.PublicKey, false));
return request.Create(issuer, NotBefore, NotBefore.AddDays(validDays), NextSerial());
}
/// <summary>A self-signed leaf that chains to nothing — the "stranger" a pin must reject.</summary>
private static X509Certificate2 SelfSignedLeaf(string commonName)
{
using var key = RSA.Create(2048);
var request = new CertificateRequest($"CN={commonName}", key, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
return request.CreateSelfSigned(NotBefore, NotBefore.AddDays(20));
}
private static byte[] NextSerial() => RandomNumberGenerator.GetBytes(8);
private static string WritePem(params X509Certificate2[] certificates)
{
using var rsa = RSA.Create(2048);
var request = new CertificateRequest("CN=otopcua-test-ca", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
request.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
using var ca = request.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddDays(1));
var path = Path.Combine(Path.GetTempPath(), $"otopcua-mqtt-ca-{Guid.NewGuid():N}.pem");
File.WriteAllText(path, ca.ExportCertificatePem());
File.WriteAllText(path, string.Join(Environment.NewLine, certificates.Select(c => c.ExportCertificatePem())));
return path;
}
/// <summary>
/// Mimics what <c>SslStream</c> hands a validation callback: the presented leaf plus a chain
/// carrying whatever intermediates the peer supplied. The chain is built (and allowed to
/// fail) purely so <c>ChainElements</c> is populated the way the platform would populate it.
/// </summary>
private static MqttClientCertificateValidationEventArgs ValidationArgs(
MqttClientOptions built,
X509Certificate2 leaf,
params X509Certificate2[] presentedIntermediates)
{
var chain = new X509Chain();
chain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllFlags;
foreach (var intermediate in presentedIntermediates)
{
chain.ChainPolicy.ExtraStore.Add(intermediate);
}
chain.Build(leaf);
return new MqttClientCertificateValidationEventArgs(
leaf,
chain,
SslPolicyErrors.RemoteCertificateChainErrors,
built.ChannelOptions);
}
/// <summary>
/// Accepts the TCP connection and then says nothing — the client sends CONNECT and waits
/// forever for a CONNACK that never comes. This is the frozen-peer shape; a closed port is