Re: [PATCH v6 3/4] tpm: add SNP SVSM vTPM driver

Stefano Garzarella
2025-04-04T18:37:58Z

On Fri, 4 Apr 2025 at 19:32, Dionna Amalie Glaze <dionnaglaze@google.com> wrote:
>
> On Thu, Apr 3, 2025 at 3:10 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> >
> > From: Stefano Garzarella <sgarzare@redhat.com>
> >
> > Add driver for the vTPM defined by the AMD SVSM spec [1].
> >
> > The specification defines a protocol that a SEV-SNP guest OS can use to
> > discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
> > in the guest context, but at a more privileged level (VMPL0).
> >
> > The new tpm-svsm platform driver uses two functions exposed by x86/sev
> > to verify that the device is actually emulated by the platform and to
> > send commands and receive responses.
> >
> > The device cannot be hot-plugged/unplugged as it is emulated by the
> > platform, so we can use module_platform_driver_probe(). The probe
> > function will only check whether in the current runtime configuration,
> > SVSM is present and provides a vTPM.
> >
> > This device does not support interrupts and sends responses to commands
> > synchronously. In order to have .recv() called just after .send() in
> > tpm_try_transmit(), the .status() callback returns 0, and both
> > .req_complete_mask and .req_complete_val are set to 0.
> >
> > [1] "Secure VM Service Module for SEV-SNP Guests"
> > Publication # 58019 Revision: 1.00
> >
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> > v6:
> > - removed the `locality` field (set to 0) and the FIXME comment [Jarkko]
> > v5:
> > - removed cancel/status/req_* ops after rebase on master that cotains
> > commit 980a573621ea ("tpm: Make chip->{status,cancel,req_canceled} opt")
> > v4:
> > - moved "asm" includes after the "linux" includes [Tom]
> > - allocated buffer separately [Tom/Jarkko/Jason]
> > v3:
> > - removed send_recv() ops and followed the ftpm driver implementing .status,
> > .req_complete_mask, .req_complete_val, etc. [Jarkko]
> > - removed link to the spec because those URLs are unstable [Borislav]
> > ---
> > drivers/char/tpm/tpm_svsm.c | 128 ++++++++++++++++++++++++++++++++++++
> > drivers/char/tpm/Kconfig | 10 +++
> > drivers/char/tpm/Makefile | 1 +
> > 3 files changed, 139 insertions(+)
> > create mode 100644 drivers/char/tpm/tpm_svsm.c
> >
> > diff --git a/drivers/char/tpm/tpm_svsm.c b/drivers/char/tpm/tpm_svsm.c
> > new file mode 100644
> > index 000000000000..b9242c9eab87
> > --- /dev/null
> > +++ b/drivers/char/tpm/tpm_svsm.c
> > @@ -0,0 +1,128 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
> > + *
> > + * Driver for the vTPM defined by the AMD SVSM spec [1].
> > + *
> > + * The specification defines a protocol that a SEV-SNP guest OS can use to
> > + * discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
> > + * in the guest context, but at a more privileged level (usually VMPL0).
> > + *
> > + * [1] "Secure VM Service Module for SEV-SNP Guests"
> > + * Publication # 58019 Revision: 1.00
> > + */
> > +
> > +#include <linux/module.h>
> > +#include <linux/kernel.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/tpm_svsm.h>
> > +
> > +#include <asm/sev.h>
> > +
> > +#include "tpm.h"
> > +
> > +struct tpm_svsm_priv {
> > + void *buffer;
> > +};
> > +
> > +static int tpm_svsm_send(struct tpm_chip *chip, u8 *buf, size_t len)
> > +{
> > + struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
> > + int ret;
> > +
> > + ret = svsm_vtpm_cmd_request_fill(priv->buffer, 0, buf, len);
> > + if (ret)
> > + return ret;
> > +
> > + /*
> > + * The SVSM call uses the same buffer for the command and for the
> > + * response, so after this call, the buffer will contain the response
> > + * that can be used by .recv() op.
> > + */
> > + return snp_svsm_vtpm_send_command(priv->buffer);
> > +}
> > +
> > +static int tpm_svsm_recv(struct tpm_chip *chip, u8 *buf, size_t len)
> > +{
> > + struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
> > +
> > + /*
> > + * The internal buffer contains the response after we send the command
> > + * to SVSM.
> > + */
> > + return svsm_vtpm_cmd_response_parse(priv->buffer, buf, len);
> > +}
> > +
> > +static struct tpm_class_ops tpm_chip_ops = {
> > + .flags = TPM_OPS_AUTO_STARTUP,
> > + .recv = tpm_svsm_recv,
> > + .send = tpm_svsm_send,
> > +};
> > +
> > +static int __init tpm_svsm_probe(struct platform_device *pdev)
> > +{
> > + struct device *dev = &pdev->dev;
> > + struct tpm_svsm_priv *priv;
> > + struct tpm_chip *chip;
> > + int err;
> > +
> > + if (!snp_svsm_vtpm_probe())
> > + return -ENODEV;
> > +
> > + priv = devm_kmalloc(dev, sizeof(*priv), GFP_KERNEL);
> > + if (!priv)
> > + return -ENOMEM;
> > +
> > + /*
> > + * The maximum buffer supported is one page (see SVSM_VTPM_MAX_BUFFER
> > + * in tpm_svsm.h).
> > + */
> > + priv->buffer = (void *)devm_get_free_pages(dev, GFP_KERNEL, 0);
> > + if (!priv->buffer)
> > + return -ENOMEM;
> > +
> > + chip = tpmm_chip_alloc(dev, &tpm_chip_ops);
> > + if (IS_ERR(chip))
> > + return PTR_ERR(chip);
> > +
> > + dev_set_drvdata(&chip->dev, priv);
> > +
> > + err = tpm2_probe(chip);
>
> Our testing is showing that tpm2_probe is hitting a null pointer deref
> in tpm_transmit.

Next time, please share a backtrace.

BTW I suspect you're not using Linus' tree, so be sure to backport
also commit 980a573621ea ("tpm: Make
chip->{status,cancel,req_canceled} opt").

Without that, you will have a null ptr dereference since .status() is
NULL from v5 of this series (as specified in the changelog).

Stefano