crypto: propagate errors from TLS session I/O callbacks

GNUTLS doesn't know how to perform I/O on anything other than plain
FDs, so the TLS session provides it with some I/O callbacks. The
GNUTLS API design requires these callbacks to return a unix errno
value, which means we're currently loosing the useful QEMU "Error"
object.

This changes the I/O callbacks in QEMU to stash the "Error" object
in the QCryptoTLSSession class, and fetch it when seeing an I/O
error returned from GNUTLS, thus preserving useful error messages.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

crypto/tlssession.c

@@ -44,6 +44,13 @@ struct QCryptoTLSSession {
     QCryptoTLSSessionReadFunc readFunc;
     void *opaque;
     char *peername;
+
+    /*
+     * Allow concurrent reads and writes, so track
+     * errors separately
+     */
+    Error *rerr;
+    Error *werr;
 };
 
 
@@ -54,6 +61,9 @@ qcrypto_tls_session_free(QCryptoTLSSession *session)
         return;
     }
 
+    error_free(session->rerr);
+    error_free(session->werr);
+
     gnutls_deinit(session->handle);
     g_free(session->hostname);
     g_free(session->peername);
@@ -67,13 +77,26 @@ static ssize_t
 qcrypto_tls_session_push(void *opaque, const void *buf, size_t len)
 {
     QCryptoTLSSession *session = opaque;
+    ssize_t ret;
 
     if (!session->writeFunc) {
         errno = EIO;
         return -1;
     };
 
-    return session->writeFunc(buf, len, session->opaque);
+    error_free(session->werr);
+    session->werr = NULL;
+
+    ret = session->writeFunc(buf, len, session->opaque, &session->werr);
+    if (ret == QCRYPTO_TLS_SESSION_ERR_BLOCK) {
+        errno = EAGAIN;
+        return -1;
+    } else if (ret < 0) {
+        errno = EIO;
+        return -1;
+    } else {
+        return ret;
+    }
 }
 
 
@@ -81,13 +104,26 @@ static ssize_t
 qcrypto_tls_session_pull(void *opaque, void *buf, size_t len)
 {
     QCryptoTLSSession *session = opaque;
+    ssize_t ret;
 
     if (!session->readFunc) {
         errno = EIO;
         return -1;
     };
 
-    return session->readFunc(buf, len, session->opaque);
+    error_free(session->rerr);
+    session->rerr = NULL;
+
+    ret = session->readFunc(buf, len, session->opaque, &session->rerr);
+    if (ret == QCRYPTO_TLS_SESSION_ERR_BLOCK) {
+        errno = EAGAIN;
+        return -1;
+    } else if (ret < 0) {
+        errno = EIO;
+        return -1;
+    } else {
+        return ret;
+    }
 }
 
 #define TLS_PRIORITY_ADDITIONAL_ANON "+ANON-DH"
@@ -450,9 +486,14 @@ qcrypto_tls_session_write(QCryptoTLSSession *session,
         if (ret == GNUTLS_E_AGAIN) {
             return QCRYPTO_TLS_SESSION_ERR_BLOCK;
         } else {
-            error_setg(errp,
-                       "Cannot write to TLS channel: %s",
-                       gnutls_strerror(ret));
+            if (session->werr) {
+                error_propagate(errp, session->werr);
+                session->werr = NULL;
+            } else {
+                error_setg(errp,
+                           "Cannot write to TLS channel: %s",
+                           gnutls_strerror(ret));
+            }
             return -1;
         }
     }
@@ -477,9 +518,14 @@ qcrypto_tls_session_read(QCryptoTLSSession *session,
                    gracefulTermination){
             return 0;
         } else {
-            error_setg(errp,
-                       "Cannot read from TLS channel: %s",
-                       gnutls_strerror(ret));
+            if (session->rerr) {
+                error_propagate(errp, session->rerr);
+                session->rerr = NULL;
+            } else {
+                error_setg(errp,
+                           "Cannot read from TLS channel: %s",
+                           gnutls_strerror(ret));
+            }
             return -1;
         }
     }
@@ -507,11 +553,21 @@ qcrypto_tls_session_handshake(QCryptoTLSSession *session,
             ret == GNUTLS_E_AGAIN) {
             ret = 1;
         } else {
-            error_setg(errp, "TLS handshake failed: %s",
-                       gnutls_strerror(ret));
+            if (session->rerr || session->werr) {
+                error_setg(errp, "TLS handshake failed: %s: %s",
+                           gnutls_strerror(ret),
+                           error_get_pretty(session->rerr ?
+                                            session->rerr : session->werr));
+            } else {
+                error_setg(errp, "TLS handshake failed: %s",
+                           gnutls_strerror(ret));
+            }
             ret = -1;
         }
     }
+    error_free(session->rerr);
+    error_free(session->werr);
+    session->rerr = session->werr = NULL;
 
     return ret;
 }

include/crypto/tlssession.h

@@ -178,12 +178,18 @@ G_DEFINE_AUTOPTR_CLEANUP_FUNC(QCryptoTLSSession, qcrypto_tls_session_free)
 int qcrypto_tls_session_check_credentials(QCryptoTLSSession *sess,
                                           Error **errp);
 
+/*
+ * These must return QCRYPTO_TLS_SESSION_ERR_BLOCK if the I/O
+ * would block, but on other errors, must fill 'errp'
+ */
 typedef ssize_t (*QCryptoTLSSessionWriteFunc)(const char *buf,
                                               size_t len,
-                                              void *opaque);
+                                              void *opaque,
+                                              Error **errp);
 typedef ssize_t (*QCryptoTLSSessionReadFunc)(char *buf,
                                              size_t len,
-                                             void *opaque);
+                                             void *opaque,
+                                             Error **errp);
 
 /**
  * qcrypto_tls_session_set_callbacks:

io/channel-tls.c

@@ -28,17 +28,16 @@
 
 static ssize_t qio_channel_tls_write_handler(const char *buf,
                                              size_t len,
-                                             void *opaque)
+                                             void *opaque,
+                                             Error **errp)
 {
     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(opaque);
     ssize_t ret;
 
-    ret = qio_channel_write(tioc->master, buf, len, NULL);
+    ret = qio_channel_write(tioc->master, buf, len, errp);
     if (ret == QIO_CHANNEL_ERR_BLOCK) {
-        errno = EAGAIN;
-        return -1;
+        return QCRYPTO_TLS_SESSION_ERR_BLOCK;
     } else if (ret < 0) {
-        errno = EIO;
         return -1;
     }
     return ret;
@@ -46,17 +45,16 @@ static ssize_t qio_channel_tls_write_handler(const char *buf,
 
 static ssize_t qio_channel_tls_read_handler(char *buf,
                                             size_t len,
-                                            void *opaque)
+                                            void *opaque,
+                                            Error **errp)
 {
     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(opaque);
     ssize_t ret;
 
-    ret = qio_channel_read(tioc->master, buf, len, NULL);
+    ret = qio_channel_read(tioc->master, buf, len, errp);
     if (ret == QIO_CHANNEL_ERR_BLOCK) {
-        errno = EAGAIN;
-        return -1;
+        return QCRYPTO_TLS_SESSION_ERR_BLOCK;
     } else if (ret < 0) {
-        errno = EIO;
         return -1;
     }
     return ret;

tests/unit/test-crypto-tlssession.c

@@ -35,18 +35,40 @@
 #define PSKFILE WORKDIR "keys.psk"
 #define KEYFILE WORKDIR "key-ctx.pem"
 
-static ssize_t testWrite(const char *buf, size_t len, void *opaque)
+static ssize_t
+testWrite(const char *buf, size_t len, void *opaque, Error **errp)
 {
     int *fd = opaque;
+    int ret;
 
-    return write(*fd, buf, len);
+    ret = write(*fd, buf, len);
+    if (ret < 0) {
+        if (errno == EAGAIN) {
+            return QCRYPTO_TLS_SESSION_ERR_BLOCK;
+        } else {
+            error_setg_errno(errp, errno, "unable to write");
+            return -1;
+        }
+    }
+    return ret;
 }
 
-static ssize_t testRead(char *buf, size_t len, void *opaque)
+static ssize_t
+testRead(char *buf, size_t len, void *opaque, Error **errp)
 {
     int *fd = opaque;
+    int ret;
 
-    return read(*fd, buf, len);
+    ret = read(*fd, buf, len);
+    if (ret < 0) {
+        if (errno == EAGAIN) {
+            return QCRYPTO_TLS_SESSION_ERR_BLOCK;
+        } else {
+            error_setg_errno(errp, errno, "unable to read");
+            return -1;
+        }
+    }
+    return ret;
 }
 
 static QCryptoTLSCreds *test_tls_creds_psk_create(