[PATCH] tpm: fix potential race condition in suspend/resume

[Sven van Ashbrook <svenva@chromium.org>]

Concurrent accesses to the tpm chip are prevented by allowing only a
single thread at a time to obtain a tpm chip reference through
tpm_try_get_ops(). However, the tpm's suspend function does not use
this mechanism, so when the tpm api is called by a kthread which
does not get frozen on suspend (such as the hw_random kthread)
it's possible that the tpm is used when already in suspend, or
in use while in the process of suspending.

This is seen on certain ChromeOS platforms - low-probability warnings
are generated during suspend. In this case, the tpm attempted to read data
from a tpm chip on an already-suspended bus.

  i2c_designware i2c_designware.1: Transfer while suspended

Fix:
1. prevent concurrent execution of tpm accesses and suspend/
   resume, by letting suspend/resume grab the tpm_mutex.
2. before commencing a tpm access, check if the tpm chip is already
   suspended. Fail with -EAGAIN if so.

Tested by running 6000 suspend/resume cycles back-to-back on a
ChromeOS "brya" device. The intermittent warnings reliably
disappear after applying this patch. No system issues were observed.

Cc: <stable@vger.kernel.org>
Fixes: e891db1a18bf ("tpm: turn on TPM on suspend for TPM 1.x")
Signed-off-by: Sven van Ashbrook <svenva@chromium.org>
---
 drivers/char/tpm/tpm-interface.c | 16 ++++++++++++++++
 include/linux/tpm.h              |  2 ++
 2 files changed, 18 insertions(+)

diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index 1621ce818705..16ca490fd483 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -82,6 +82,11 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
 		return -E2BIG;
 	}
 
+	if (chip->is_suspended) {
+		dev_info(&chip->dev, "blocking transmit while suspended\n");
+		return -EAGAIN;
+	}
+
 	rc = chip->ops->send(chip, buf, count);
 	if (rc < 0) {
 		if (rc != -EPIPE)
@@ -394,6 +399,8 @@ int tpm_pm_suspend(struct device *dev)
 	if (!chip)
 		return -ENODEV;
 
+	mutex_lock(&chip->tpm_mutex);
+
 	if (chip->flags & TPM_CHIP_FLAG_ALWAYS_POWERED)
 		goto suspended;
 
@@ -411,6 +418,11 @@ int tpm_pm_suspend(struct device *dev)
 	}
 
 suspended:
+	if (!rc)
+		chip->is_suspended = true;
+
+	mutex_unlock(&chip->tpm_mutex);
+
 	return rc;
 }
 EXPORT_SYMBOL_GPL(tpm_pm_suspend);
@@ -426,6 +438,10 @@ int tpm_pm_resume(struct device *dev)
 	if (chip == NULL)
 		return -ENODEV;
 
+	mutex_lock(&chip->tpm_mutex);
+	chip->is_suspended = false;
+	mutex_unlock(&chip->tpm_mutex);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(tpm_pm_resume);
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index d7c67581929f..0fbc1a43ae80 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -131,6 +131,8 @@ struct tpm_chip {
 	int dev_num;		/* /dev/tpm# */
 	unsigned long is_open;	/* only one allowed */
 
+	bool is_suspended;
+
 	char hwrng_name[64];
 	struct hwrng hwrng;
 
-- 
2.37.1.559.g78731f0fdb-goog