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

From: Jarkko Sakkinen <[email-masked]>
Date: Tue, 21 Dec 2021 10:47:28 +0200

On Sat, Dec 11, 2021 at 08:28:04PM -0500, 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.
> 
> Fixes: 39d0099f9439 ("powerpc/pseries: Add shutdown() to vio_driver and vio_bus")
> Signed-off-by: Stefan Berger <[email-masked]>
> ---
>  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;

This triggers to all drivers, not just tpm_ibmvtpm, i.e. the fix has
side-effects.

/Jarkko