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

[Stefan Berger <stefanb@linux.ibm.com>]

On 12/21/21 03:47, Jarkko Sakkinen wrote:
> 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 <stefanb@linux.ibm.com>
>> ---
>>   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.

What are those side-effects?

   Stefan

>
> /Jarkko
>