Re: [PATCH v7 2/2] tpm: add backend for mssim

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, Sep 27, 2023 at 12:49:08PM -0400, James Bottomley wrote:
>> From: James Bottomley <James.Bottomley@HansenPartnership.com>
>> 
>> The Microsoft Simulator (mssim) is the reference emulation platform
>> for the TCG TPM 2.0 specification.
>> 
>> https://github.com/Microsoft/ms-tpm-20-ref.git
>> 
>> It exports a fairly simple network socket based protocol on two
>> sockets, one for command (default 2321) and one for control (default
>> 2322).  This patch adds a simple backend that can speak the mssim
>> protocol over the network.  It also allows the two sockets to be
>> specified on the command line.  The benefits are twofold: firstly it
>> gives us a backend that actually speaks a standard TPM emulation
>> protocol instead of the linux specific TPM driver format of the
>> current emulated TPM backend and secondly, using the microsoft
>> protocol, the end point of the emulator can be anywhere on the
>> network, facilitating the cloud use case where a central TPM service
>> can be used over a control network.
>> 
>> The implementation does basic control commands like power off/on, but
>> doesn't implement cancellation or startup.  The former because
>> cancellation is pretty much useless on a fast operating TPM emulator
>> and the latter because this emulator is designed to be used with OVMF
>> which itself does TPM startup and I wanted to validate that.
>> 
>> To run this, simply download an emulator based on the MS specification
>> (package ibmswtpm2 on openSUSE) and run it, then add these two lines
>> to the qemu command and it will use the emulator.
>> 
>>     -tpmdev mssim,id=tpm0 \
>>     -device tpm-crb,tpmdev=tpm0 \
>> 
>> to use a remote emulator replace the first line with
>> 
>>     -tpmdev "{'type':'mssim','id':'tpm0','command':{'type':inet,'host':'remote','port':'2321'}}"
>> 
>> tpm-tis also works as the backend.
>> 
>> Signed-off-by: James Bottomley <jejb@linux.ibm.com>
>> Acked-by: Markus Armbruster <armbru@redhat.com>

[...]

>> diff --git a/backends/tpm/tpm_mssim.c b/backends/tpm/tpm_mssim.c
>> new file mode 100644
>> index 0000000000..b8a12dce04
>> --- /dev/null
>> +++ b/backends/tpm/tpm_mssim.c
>> @@ -0,0 +1,290 @@
>> +/*
>> + * Emulator TPM driver which connects over the mssim protocol
>> + * SPDX-License-Identifier: GPL-2.0-or-later
>> + *
>> + * Copyright (c) 2022
>> + * Author: James Bottomley <jejb@linux.ibm.com>
>> + */
>> +
>> +#include "qemu/osdep.h"
>> +#include "qemu/error-report.h"
>> +#include "qemu/sockets.h"
>> +
>> +#include "qapi/clone-visitor.h"
>> +#include "qapi/qapi-visit-tpm.h"
>> +
>> +#include "io/channel-socket.h"
>> +
>> +#include "sysemu/runstate.h"
>> +#include "sysemu/tpm_backend.h"
>> +#include "sysemu/tpm_util.h"
>> +
>> +#include "qom/object.h"
>> +
>> +#include "tpm_int.h"
>> +#include "tpm_mssim.h"
>> +
>> +#define ERROR_PREFIX "TPM mssim Emulator: "
>> +
>> +#define TYPE_TPM_MSSIM "tpm-mssim"
>> +OBJECT_DECLARE_SIMPLE_TYPE(TPMMssim, TPM_MSSIM)
>> +
>> +struct TPMMssim {
>> +    TPMBackend parent;
>> +
>> +    TPMMssimOptions opts;
>> +
>> +    QIOChannelSocket *cmd_qc, *ctrl_qc;
>> +};
>> +
>> +static int tpm_send_ctrl(TPMMssim *t, uint32_t cmd, Error **errp)
>> +{
>> +    int ret;
>> +
>> +    qio_channel_socket_connect_sync(t->ctrl_qc, t->opts.control, errp);
>
> Need to assign to 'ret' and check for failure here, otherwise the
> next call to write_all will overwrite the useful message in 'errp'
> with a less helpful one.

No, it'll crash :)

An @errp argument must point to a null pointer.  If it doesn't, setting
an error will trip error_setv()'s assertion.

> +    cmd = htonl(cmd);
> +    ret = qio_channel_write_all(QIO_CHANNEL(t->ctrl_qc),
> +                                (char *)&cmd, sizeof(cmd), errp);
> +    if (ret != 0) {
> +        goto out;
> +    }

qapi/error.h's big comment advises:

 * Receive and accumulate multiple errors (first one wins):
 *     Error *err = NULL, *local_err = NULL;
 *     foo(arg, &err);
 *     bar(arg, &local_err);
 *     error_propagate(&err, local_err);
 *     if (err) {
 *         handle the error...
 *     }
 *
 * Do *not* "optimize" this to
 *     Error *err = NULL;
 *     foo(arg, &err);
 *     bar(arg, &err); // WRONG!
 *     if (err) {
 *         handle the error...
 *     }
 * because this may pass a non-null err to bar().
 *
 * Likewise, do *not*
 *     Error *err = NULL;
 *     if (cond1) {
 *         error_setg(&err, ...);
 *     }
 *     if (cond2) {
 *         error_setg(&err, ...); // WRONG!
 *     }
 * because this may pass a non-null err to error_setg().

The quoted code is like the last example, except the error_setg() lurk
within the functions called.

[...]