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

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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) {




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