Fix severe memory leaks in GSSAPI encryption support.

Both the backend and libpq leaked buffers containing encrypted data
to be transmitted, so that the process size would grow roughly as
the total amount of data sent.

There were also far-less-critical leaks of the same sort in GSSAPI
session establishment.

Oversight in commit b0b39f72, which I failed to notice while
reviewing the code in 2c0cdc81.

Per complaint from pmc@citylink.
Back-patch to v12 where this code was introduced.

Discussion: https://postgr.es/m/20200504115649.GA77072@gate.oper.dinoex.org

src/backend/libpq/be-secure-gssapi.c

@@ -215,6 +215,9 @@ be_gssapi_write(Port *port, void *ptr, size_t len)
 
 		memcpy(PqGSSSendBuffer + PqGSSSendLength, output.value, output.length);
 		PqGSSSendLength += output.length;
+
+		/* Release buffer storage allocated by GSSAPI */
+		gss_release_buffer(&minor, &output);
 	}
 
 	/* If we get here, our counters should all match up. */
@@ -371,6 +374,7 @@ be_gssapi_read(Port *port, void *ptr, size_t len)
 		/* Our receive buffer is now empty, reset it */
 		PqGSSRecvLength = 0;
 
+		/* Release buffer storage allocated by GSSAPI */
 		gss_release_buffer(&minor, &output);
 	}
 
@@ -590,7 +594,10 @@ secure_open_gssapi(Port *port)
 				 */
 				if (ret < 0 &&
 					!(errno == EWOULDBLOCK || errno == EAGAIN || errno == EINTR))
+				{
+					gss_release_buffer(&minor, &output);
 					return -1;
+				}
 
 				/* Wait and retry if we couldn't write yet */
 				if (ret <= 0)

src/interfaces/libpq/fe-secure-gssapi.c

@@ -232,6 +232,9 @@ pg_GSS_write(PGconn *conn, const void *ptr, size_t len)
 
 		memcpy(PqGSSSendBuffer + PqGSSSendLength, output.value, output.length);
 		PqGSSSendLength += output.length;
+
+		/* Release buffer storage allocated by GSSAPI */
+		gss_release_buffer(&minor, &output);
 	}
 
 	/* If we get here, our counters should all match up. */
@@ -241,6 +244,7 @@ pg_GSS_write(PGconn *conn, const void *ptr, size_t len)
 	ret = bytes_sent;
 
 cleanup:
+	/* Release GSSAPI buffer storage, if we didn't already */
 	if (output.value != NULL)
 		gss_release_buffer(&minor, &output);
 	return ret;
@@ -408,12 +412,14 @@ pg_GSS_read(PGconn *conn, void *ptr, size_t len)
 		/* Our receive buffer is now empty, reset it */
 		PqGSSRecvLength = 0;
 
+		/* Release buffer storage allocated by GSSAPI */
 		gss_release_buffer(&minor, &output);
 	}
 
 	ret = bytes_returned;
 
 cleanup:
+	/* Release GSSAPI buffer storage, if we didn't already */
 	if (output.value != NULL)
 		gss_release_buffer(&minor, &output);
 	return ret;
@@ -652,6 +658,7 @@ pqsecure_open_gss(PGconn *conn)
 		gss_release_cred(&minor, &conn->gcred);
 		conn->gcred = GSS_C_NO_CREDENTIAL;
 		conn->gssenc = true;
+		gss_release_buffer(&minor, &output);
 
 		/*
 		 * Determine the max packet size which will fit in our buffer, after
@@ -676,6 +683,7 @@ pqsecure_open_gss(PGconn *conn)
 	{
 		pg_GSS_error(libpq_gettext("GSSAPI context establishment error"),
 					 conn, major, minor);
+		gss_release_buffer(&minor, &output);
 		return PGRES_POLLING_FAILED;
 	}
 
@@ -690,6 +698,7 @@ pqsecure_open_gss(PGconn *conn)
 
 	/* We don't bother with PqGSSSendConsumed here */
 
+	/* Release buffer storage allocated by GSSAPI */
 	gss_release_buffer(&minor, &output);
 
 	/* Ask to be called again to write data */