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Message-ID: <CAGxU2F7NDvHPhyHbN+dBjVta2NLstyoV2SQ6VkwDTf2FiKLasA@mail.gmail.com>
Date: Wed, 9 Apr 2025 11:45:27 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Dionna Amalie Glaze <dionnaglaze@...gle.com>
Cc: Jarkko Sakkinen <jarkko@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>, "H. Peter Anvin" <hpa@...or.com>,
linux-coco@...ts.linux.dev, linux-integrity@...r.kernel.org,
Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Tom Lendacky <thomas.lendacky@....com>, Joerg Roedel <jroedel@...e.de>,
Claudio Carvalho <cclaudio@...ux.ibm.com>,
James Bottomley <James.Bottomley@...senpartnership.com>, linux-kernel@...r.kernel.org,
Dov Murik <dovmurik@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v6 3/4] tpm: add SNP SVSM vTPM driver
On Fri, 4 Apr 2025 at 20:59, Dionna Amalie Glaze <dionnaglaze@...gle.com> wrote:
>
> On Fri, Apr 4, 2025 at 11:37 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
> >
> > On Fri, 4 Apr 2025 at 19:32, Dionna Amalie Glaze <dionnaglaze@...gle.com> wrote:
> > >
> > > On Thu, Apr 3, 2025 at 3:10 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
> > > >
> > > > From: Stefano Garzarella <sgarzare@...hat.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@...hat.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.
>
> Right, my bad.
> >
> > 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).
>
> Thanks, I missed that detail. Will report back.
I'm preparing v7, is your issue solved or do we need to investigate
better before sending a v7?
Thanks,
Stefano
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