[PATCH 2/2] tpm: Fix crash on tmprm release

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

If the tpm_tis module is removed (or a system shutdown is triggered)
while the tpmrm device is use, the kernel crashes due to chip->ops being
NULL:

 # exec 3<>/dev/tpmrm0
 # rmmod tpm_tis
 # exit
 ==================================================================
 BUG: KASAN: null-ptr-deref in tpm_chip_start+0x2d/0x120 [tpm]
 Read of size 8 at addr 0000000000000060 by task sh/994

 Call Trace:
  kasan_report.cold.13+0x10f/0x111
  tpm_chip_start+0x2d/0x120 [tpm]
  tpm2_del_space+0x2c/0xa0 [tpm]
  tpmrm_release+0x3f/0x50 [tpm]
  __fput+0x110/0x3c0
  task_work_run+0x94/0xd0
  do_exit+0x683/0x13e0
  do_group_exit+0x8b/0x140
  do_syscall_64+0x3c/0x80
 ==================================================================

Fix this by making tpm2_del_space() use tpm_try_get_ops().  The latter
already includes the calls to tpm_chip_start() and tpm_chip_stop().

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
 drivers/char/tpm/tpm2-space.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
index 784b8b3cb903..e1111261021f 100644
--- a/drivers/char/tpm/tpm2-space.c
+++ b/drivers/char/tpm/tpm2-space.c
@@ -58,12 +58,10 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
 
 void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
 {
-	mutex_lock(&chip->tpm_mutex);
-	if (!tpm_chip_start(chip)) {
+	if (!tpm_try_get_ops(chip)) {
 		tpm2_flush_sessions(chip, space);
-		tpm_chip_stop(chip);
+		tpm_put_ops(chip);
 	}
-	mutex_unlock(&chip->tpm_mutex);
 	kfree(space->context_buf);
 	kfree(space->session_buf);
 }
-- 
2.28.0