[PATCH] tpm: Disable hwrng for TPM 1 if PM_SLEEP is enabled

[Jason A. Donenfeld]

TPM 1's support for its hardware RNG is broken across system suspends,
due to races or locking issues or something else that haven't been
diagnosed or fixed yet. These issues prevent the system from actually
suspending. So disable the driver in this case. Later, when this is
fixed properly, we can remove this.

Current breakage amounts to something like:

  tpm tpm0: A TPM error (28) occurred continue selftest
  ...
  tpm tpm0: A TPM error (28) occurred attempting get random
  ...
  tpm tpm0: Error (28) sending savestate before suspend
  tpm_tis 00:08: PM: __pnp_bus_suspend(): tpm_pm_suspend+0x0/0x80 returns 28
  tpm_tis 00:08: PM: dpm_run_callback(): pnp_bus_suspend+0x0/0x10 returns 28
  tpm_tis 00:08: PM: failed to suspend: error 28
  PM: Some devices failed to suspend, or early wake event detected

This issue was partially fixed by 23393c646142 ("char: tpm: Protect
tpm_pm_suspend with locks"), in a last minute 6.1 commit that Linus took
directly because the TPM maintainers weren't available. However, it
seems like this just addresses the most common cases of the bug, rather
than addressing it entirely. So there are more things to fix still,
apparently.

The hwrng driver appears already to be occasionally disabled due to
other conditions, so this shouldn't be too large of a surprise.

Link: https://lore.kernel.org/lkml/7cbe96cf-e0b5-ba63-d1b4-f63d2e826efa@xxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx # 6.1+
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 drivers/char/tpm/tpm-chip.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 741d8f3e8fb3..eed67ea8d3a7 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -524,6 +524,14 @@ static int tpm_add_hwrng(struct tpm_chip *chip)
 	if (!IS_ENABLED(CONFIG_HW_RANDOM_TPM) || tpm_is_firmware_upgrade(chip))
 		return 0;
 
+	/*
+	 * This driver's support for using the RNG across suspend is broken on
+	 * TPM1. Until somebody fixes this, just stop registering a HWRNG in
+	 * that case.
+	 */
+	if (!(chip->flags & TPM_CHIP_FLAG_TPM2) && IS_ENABLED(CONFIG_PM_SLEEP))
+		return 0;
+
 	snprintf(chip->hwrng_name, sizeof(chip->hwrng_name),
 		 "tpm-rng-%d", chip->dev_num);
 	chip->hwrng.name = chip->hwrng_name;
-- 
2.39.0