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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Dec 21, 2021 at 09:01:06AM -0500, Stefan Berger wrote:
> 
> 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?

It does change behaviour for all drivers, which is not acceptable for a
bug fix.

/Jarkko