[PATCH 3/3] tpm: Provide strong locking for device removal

From: Jason Gunthorpe
Date: Fri Feb 12 2016 - 22:29:53 EST

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Add a read/write semaphore around the ops function pointers so
ops can be set to null when the driver un-registers.

Previously the tpm core expected module locking to be enough to
ensure that tpm_unregister could not be called during certain times,
however that hasn't been sufficient for a long time.

Introduce a read/write semaphore around 'ops' so the core can set
it to null when unregistering. This provides a strong fence around
the driver callbacks, guaranteeing to the driver that no callbacks
are running or will run again.

For now the ops_lock is placed very high in the call stack, it could
be pushed down and made more granular in future if necessary.

Signed-off-by: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx>
---
drivers/char/tpm/tpm-chip.c | 72 ++++++++++++++++++++++++++++++++++++----
drivers/char/tpm/tpm-dev.c | 10 +++++-
drivers/char/tpm/tpm-interface.c | 18 +++++-----
drivers/char/tpm/tpm-sysfs.c | 5 +++
drivers/char/tpm/tpm.h | 14 +++++---
5 files changed, 98 insertions(+), 21 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index df4132e8b982..647fdf327537 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -36,9 +36,59 @@ static DEFINE_SPINLOCK(driver_lock);
struct class *tpm_class;
dev_t tpm_devt;

-/*
- * tpm_chip_find_get - return tpm_chip for a given chip number
- * @chip_num the device number for the chip
+/**
+ * tpm_try_get_ops() - Get a ref to the tpm_chip
+ * @chip: Chip to ref
+ *
+ * The caller must already have some kind of locking to ensure that chip is
+ * valid. This function will lock the chip so that the ops member can be
+ * accessed safely. The locking prevents tpm_chip_unregister from
+ * completing, so it should not be held for long periods.
+ *
+ * Returns -ERRNO if the chip could not be got.
+ */
+int tpm_try_get_ops(struct tpm_chip *chip)
+{
+ int rc = -EIO;
+
+ get_device(&chip->dev);
+
+ down_read(&chip->ops_sem);
+ if (!chip->ops)
+ goto out_lock;
+
+ if (!try_module_get(chip->dev.parent->driver->owner))
+ goto out_lock;
+
+ return 0;
+out_lock:
+ up_read(&chip->ops_sem);
+ put_device(&chip->dev);
+ return rc;
+}
+EXPORT_SYMBOL_GPL(tpm_try_get_ops);
+
+/**
+ * tpm_put_ops() - Release a ref to the tpm_chip
+ * @chip: Chip to put
+ *
+ * This is the opposite pair to tpm_try_get_ops(). After this returns chip may
+ * be kfree'd.
+ */
+void tpm_put_ops(struct tpm_chip *chip)
+{
+ module_put(chip->dev.parent->driver->owner);
+ up_read(&chip->ops_sem);
+ put_device(&chip->dev);
+}
+EXPORT_SYMBOL_GPL(tpm_put_ops);
+
+/**
+ * tpm_chip_find_get() - return tpm_chip for a given chip number
+ * @chip_num: id to find
+ *
+ * The return'd chip has been tpm_try_get_ops'd and must be released via
+ * tpm_put_ops
*/
struct tpm_chip *tpm_chip_find_get(int chip_num)
{
@@ -49,10 +99,10 @@ struct tpm_chip *tpm_chip_find_get(int chip_num)
if (chip_num != TPM_ANY_NUM && chip_num != pos->dev_num)
continue;

- if (try_module_get(pos->dev.parent->driver->owner)) {
+ /* rcu prevents chip from being free'd */
+ if (!tpm_try_get_ops(pos))
chip = pos;
- break;
- }
+ break;
}
rcu_read_unlock();
return chip;
@@ -95,6 +145,7 @@ struct tpm_chip *tpmm_chip_alloc(struct device *dev,
return ERR_PTR(-ENOMEM);

mutex_init(&chip->tpm_mutex);
+ init_rwsem(&chip->ops_sem);
INIT_LIST_HEAD(&chip->list);

chip->ops = ops;
@@ -174,6 +225,12 @@ static int tpm_dev_add_device(struct tpm_chip *chip)
static void tpm_dev_del_device(struct tpm_chip *chip)
{
cdev_del(&chip->cdev);
+
+ /* Make the driver uncallable. */
+ down_write(&chip->ops_sem);
+ chip->ops = NULL;
+ up_write(&chip->ops_sem);
+
device_unregister(&chip->dev);
}

@@ -259,6 +316,9 @@ EXPORT_SYMBOL_GPL(tpm_chip_register);
* Takes the chip first away from the list of available TPM chips and then
* cleans up all the resources reserved by tpm_chip_register().
*
+ * Once this function returns the driver call backs in 'op's will not be
+ * running and will no longer start.
+ *
* NOTE: This function should be only called before deinitializing chip
* resources.
*/
diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
index 4009765c14fd..1474f8b8297b 100644
--- a/drivers/char/tpm/tpm-dev.c
+++ b/drivers/char/tpm/tpm-dev.c
@@ -136,9 +136,17 @@ static ssize_t tpm_write(struct file *file, const char __user *buf,
return -EFAULT;
}

- /* atomic tpm command send and result receive */
+ /* atomic tpm command send and result receive. We only hold the ops
+ * lock during this period so that the tpm can be unregistered even if
+ * the char dev is held open.
+ */
+ if (tpm_try_get_ops(priv->chip)) {
+ mutex_unlock(&priv->buffer_mutex);
+ return -EPIPE;
+ }
out_size = tpm_transmit(priv->chip, priv->data_buffer,
sizeof(priv->data_buffer));
+ tpm_put_ops(priv->chip);
if (out_size < 0) {
mutex_unlock(&priv->buffer_mutex);
return out_size;
diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index 483f86ff6a0a..49bdab5509b2 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -700,7 +700,7 @@ int tpm_is_tpm2(u32 chip_num)

rc = (chip->flags & TPM_CHIP_FLAG_TPM2) != 0;

- tpm_chip_put(chip);
+ tpm_put_ops(chip);

return rc;
}
@@ -729,7 +729,7 @@ int tpm_pcr_read(u32 chip_num, int pcr_idx, u8 *res_buf)
rc = tpm2_pcr_read(chip, pcr_idx, res_buf);
else
rc = tpm_pcr_read_dev(chip, pcr_idx, res_buf);
- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}
EXPORT_SYMBOL_GPL(tpm_pcr_read);
@@ -764,7 +764,7 @@ int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash)

if (chip->flags & TPM_CHIP_FLAG_TPM2) {
rc = tpm2_pcr_extend(chip, pcr_idx, hash);
- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}

@@ -774,7 +774,7 @@ int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash)
rc = tpm_transmit_cmd(chip, &cmd, EXTEND_PCR_RESULT_SIZE,
"attempting extend a PCR value");

- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}
EXPORT_SYMBOL_GPL(tpm_pcr_extend);
@@ -855,7 +855,7 @@ int tpm_send(u32 chip_num, void *cmd, size_t buflen)

rc = tpm_transmit_cmd(chip, cmd, buflen, "attempting tpm_cmd");

- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}
EXPORT_SYMBOL_GPL(tpm_send);
@@ -1037,7 +1037,7 @@ int tpm_get_random(u32 chip_num, u8 *out, size_t max)

if (chip->flags & TPM_CHIP_FLAG_TPM2) {
err = tpm2_get_random(chip, out, max);
- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return err;
}

@@ -1059,7 +1059,7 @@ int tpm_get_random(u32 chip_num, u8 *out, size_t max)
num_bytes -= recd;
} while (retries-- && total < max);

- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return total ? total : -EIO;
}
EXPORT_SYMBOL_GPL(tpm_get_random);
@@ -1085,7 +1085,7 @@ int tpm_seal_trusted(u32 chip_num, struct trusted_key_payload *payload,

rc = tpm2_seal_trusted(chip, payload, options);

- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}
EXPORT_SYMBOL_GPL(tpm_seal_trusted);
@@ -1111,7 +1111,7 @@ int tpm_unseal_trusted(u32 chip_num, struct trusted_key_payload *payload,

rc = tpm2_unseal_trusted(chip, payload, options);

- tpm_chip_put(chip);
+ tpm_put_ops(chip);
return rc;
}
EXPORT_SYMBOL_GPL(tpm_unseal_trusted);
diff --git a/drivers/char/tpm/tpm-sysfs.c b/drivers/char/tpm/tpm-sysfs.c
index d93736aa2703..34e7fc7e590c 100644
--- a/drivers/char/tpm/tpm-sysfs.c
+++ b/drivers/char/tpm/tpm-sysfs.c
@@ -295,5 +295,10 @@ int tpm_sysfs_add_device(struct tpm_chip *chip)

void tpm_sysfs_del_device(struct tpm_chip *chip)
{
+ /* The sysfs routines rely on an implicit tpm_try_get_ops, this
+ * function is called before ops is null'd and the sysfs core
+ * synchronizes this removal so that no callbacks are running or can
+ * run again
+ */
sysfs_remove_group(&chip->dev.parent->kobj, &tpm_dev_group);
}
diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
index 1e097350ce79..026695ae44ca 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -177,7 +177,13 @@ struct tpm_chip {
struct device dev;
struct cdev cdev;

+ /* A driver callback under ops cannot be run unless ops_sem is held
+ * (sometimes implicitly, eg for the sysfs code). ops becomes null
+ * when the driver is unregistered, see tpm_try_get_ops.
+ */
+ struct rw_semaphore ops_sem;
const struct tpm_class_ops *ops;
+
unsigned int flags;

int dev_num; /* /dev/tpm# */
@@ -202,11 +208,6 @@ struct tpm_chip {

#define to_tpm_chip(d) container_of(d, struct tpm_chip, dev)

-static inline void tpm_chip_put(struct tpm_chip *chip)
-{
- module_put(chip->dev.parent->driver->owner);
-}
-
static inline int tpm_read_index(int base, int index)
{
outb(index, base);
@@ -514,6 +515,9 @@ extern int wait_for_tpm_stat(struct tpm_chip *, u8, unsigned long,
wait_queue_head_t *, bool);

struct tpm_chip *tpm_chip_find_get(int chip_num);
+__must_check int tpm_try_get_ops(struct tpm_chip *chip);
+void tpm_put_ops(struct tpm_chip *chip);
+
extern struct tpm_chip *tpmm_chip_alloc(struct device *dev,
const struct tpm_class_ops *ops);
extern int tpm_chip_register(struct tpm_chip *chip);
--
1.9.1

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