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

[Stefan Berger]

On 12/21/21 09:01, 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@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;
> > This triggers to all drivers, not just tpm_ibmvtpm, i.e. the fix has
> > side-effects.
>
> What are those side-effects?
I am asking because if one entered tpm_chip_start() with chip->ops = 
NULL it would crash any system. So now the side-effect is that one can 
call this function without crashing the system but gets an -EINVAL back.

Another alternative that prevents these crashes is this change here 
including code deduplication:

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ddaeceb7e109..888d37293091 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -296,7 +296,7 @@ static int tpm_class_shutdown(struct device *dev)
        struct tpm_chip *chip = container_of(dev, struct tpm_chip, dev);

        down_write(&chip->ops_sem);
-       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
+       if (chip->ops && chip->flags & TPM_CHIP_FLAG_TPM2) {
                if (!tpm_chip_start(chip)) {
                        tpm2_shutdown(chip, TPM2_SU_CLEAR);
                        tpm_chip_stop(chip);
@@ -473,15 +473,7 @@ 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);
+       tpm_class_shutdown(&chip->dev);
 }

 static void tpm_del_legacy_sysfs(struct tpm_chip *chip)



    Stefan