[PATCH] tpm: Fix null pointer dereference on chip register error path

[Milan Broz <gmazyland@xxxxxxxxx>]

If clk_enable is not defined and chip initialization
is canceled code hits null dereference.

Easily reproducible with vTPM init fail:
  swtpm chardev --tpmstate dir=nonexistent_dir --tpm2 --vtpm-proxy

BUG: kernel NULL pointer dereference, address: 00000000
...
Call Trace:
 tpm_chip_start+0x9d/0xa0 [tpm]
 tpm_chip_register+0x10/0x1a0 [tpm]
 vtpm_proxy_work+0x11/0x30 [tpm_vtpm_proxy]
 process_one_work+0x214/0x5a0
 worker_thread+0x134/0x3e0
 ? process_one_work+0x5a0/0x5a0
 kthread+0xd4/0x100
 ? process_one_work+0x5a0/0x5a0
 ? kthread_park+0x90/0x90
 ret_from_fork+0x19/0x24

Signed-off-by: Milan Broz <gmazyland@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/char/tpm/tpm-chip.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 90325e1749fb..4c2af643d698 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -95,7 +95,8 @@ int tpm_chip_start(struct tpm_chip *chip)
 	if (chip->locality == -1) {
 		ret = tpm_request_locality(chip);
 		if (ret) {
-			chip->ops->clk_enable(chip, false);
+			if (chip->ops->clk_enable)
+				chip->ops->clk_enable(chip, false);
 			return ret;
 		}
 	}
-- 
2.20.1