Re: [PATCH] tpm: Fix kexec crash due to access to ops NULL pointer (powerpc)

[Stefan Berger]

On 12/20/21 19:39, Tyrel Datwyler wrote:
> On 12/11/21 5:28 PM, Stefan Berger wrote:
> > Fix the following crash on kexec by checking chip->ops for a NULL pointer
> > in tpm_chip_start() and returning an error code if this is the case.
> >
> > BUG: Kernel NULL pointer dereference on read at 0x00000060
> > Faulting instruction address: 0xc00000000099a06c
> > Oops: Kernel access of bad area, sig: 11 [#1]
> > ...
> > NIP [c00000000099a06c] tpm_chip_start+0x2c/0x140
> >   LR [c00000000099a808] tpm_chip_unregister+0x108/0x170
> > Call Trace:
> > [c0000000188bfa00] [c000000002b03930] fw_devlink_strict+0x0/0x8 (unreliable)
> > [c0000000188bfa30] [c00000000099a808] tpm_chip_unregister+0x108/0x170
> > [c0000000188bfa70] [c0000000009a3874] tpm_ibmvtpm_remove+0x34/0x130
> > [c0000000188bfae0] [c000000000110dbc] vio_bus_remove+0x5c/0xb0
> > [c0000000188bfb20] [c0000000009bc154] device_shutdown+0x1d4/0x3a8
> > [c0000000188bfbc0] [c000000000196e14] kernel_restart_prepare+0x54/0x70
> >
> > The referenced patch below introduced a function to shut down the VIO bus.
> > The bus shutdown now calls tpm_del_char_device (via tpm_chip_unregister)
> > after a call to tpm_class_shutdown, which already set chip->ops to NULL.
> > The crash occurrs when tpm_del_char_device calls tpm_chip_start with the
> > chip->ops NULL pointer.
> It was unclear to me at first, but IIUC the problem is the ibmvtpm device
> receives two shutdown calls, the first is a class shutdown call for TPM devices,
> followed by a bus shutdown call for VIO devices.
>
> It appears that the class shutdown routines are meant to be a pre-shutdown
> routine as they are defined as class->shutdown_pre(), and it is clearly allowed
> to call class->shutdown_pre() followed by one of but not both of the following
> bus->shutdown() or driver->shutdown(). Even tpm_class_shutdown() mentions in the
> function comment that bus/device shutdown to follow.

I suppose you are referring to this here:

/**
 * tpm_class_shutdown() - prepare the TPM device for loss of power.
 * @dev: device to which the chip is associated.
 *
 * Issues a TPM2_Shutdown command prior to loss of power, as required 
by the
 * TPM 2.0 spec. Then, calls bus- and device- specific shutdown code.
 *
 * Return: always 0 (i.e. success)
 */

I think this comment still refers to the ancient code here:

https://elixir.bootlin.com/linux/v4.4.295/source/drivers/char/tpm/tpm-chip.c#L161

    if (dev->bus && dev->bus->shutdown)
        dev->bus->shutdown(dev);
    else if (dev->driver && dev->driver->shutdown)
        dev->driver->shutdown(dev);



> > Fixes: 39d0099f9439 ("powerpc/pseries: Add shutdown() to vio_driver and vio_bus")
> This patch left implementing each vio driver shutdown routine as an exercise for
> the respective maintainers, and instead just big hammers anything that doesn't
> have a shutdown routine by calling the driver->remove().
>
> If tpm_class_shutdown() quiecses ibmvtpm enough implementing a no-op
> ibmvtpm->shutdown() with a comment saying so suffices.
>
> However, the generic TPM code is still introducing a bug that an attempt to call
> tpm_chip_unregister() after tpm_class_shutdown() will crash as mentioned above.


An alternative solution may be this here:

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ddaeceb7e109..4cb908349b31 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -473,15 +473,8 @@ static void tpm_del_char_device(struct tpm_chip *chip)
        mutex_unlock(&idr_lock);

        /* Make the driver uncallable. */
-       down_write(&chip->ops_sem);
-       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
-               if (!tpm_chip_start(chip)) {
-                       tpm2_shutdown(chip, TPM2_SU_CLEAR);
-                       tpm_chip_stop(chip);
-               }
-       }
-       chip->ops = NULL;
-       up_write(&chip->ops_sem);
+       if (chip->ops)
+               tpm_class_shutdown(&chip->dev);
 }

 static void tpm_del_legacy_sysfs(struct tpm_chip *chip)


I could post this as v2 ?! Let me know...

   Stefan


> > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > ---
> >   drivers/char/tpm/tpm-chip.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> > index ddaeceb7e109..cca1bde296ee 100644
> > --- a/drivers/char/tpm/tpm-chip.c
> > +++ b/drivers/char/tpm/tpm-chip.c
> > @@ -101,6 +101,9 @@ int tpm_chip_start(struct tpm_chip *chip)
> >   {
> >   	int ret;
> >
> > +	if (!chip->ops)
> > +		return -EINVAL;
> > +
> I wonder if a better fix would to have tpm_del_char_device() check for valid
> chip->ops and call tpm_class_shutdown() when the ops are still valid. Calling
> tpm_class_shutdown() allows for some code deduplication in tpm_del_char_device().
>
> -Tyrel
>
> >   	tpm_clk_enable(chip);
> >
> >   	if (chip->locality == -1) {