[PATCH 3.16 095/366] tpm: fix race condition in tpm_common_write()

Ben Hutchings
2018-11-11T20:39:44Z

From: Tadeusz Struk <tadeusz.struk@intel.com>

commit 3ab2011ea368ec3433ad49e1b9e1c7b70d2e65df upstream.

There is a race condition in tpm_common_write function allowing
two threads on the same /dev/tpm<N>, or two different applications
on the same /dev/tpmrm<N> to overwrite each other commands/responses.
Fixed this by taking the priv->buffer_mutex early in the function.

Also converted the priv->data_pending from atomic to a regular size_t
type. There is no need for it to be atomic since it is only touched
under the protection of the priv->buffer_mutex.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
[bwh: Backported to 3.16: adjust filenames, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/drivers/char/tpm/tpm-dev.c
+++ b/drivers/char/tpm/tpm-dev.c
@@ -26,7 +26,7 @@ struct file_priv {
struct tpm_chip *chip;

/* Data passed to and from the tpm via the read/write calls */
- atomic_t data_pending;
+ size_t data_pending;
struct mutex buffer_mutex;

struct timer_list user_read_timer; /* user needs to claim result */
@@ -47,7 +47,7 @@ static void timeout_work(struct work_str
struct file_priv *priv = container_of(work, struct file_priv, work);

mutex_lock(&priv->buffer_mutex);
- atomic_set(&priv->data_pending, 0);
+ priv->data_pending = 0;
memset(priv->data_buffer, 0, sizeof(priv->data_buffer));
mutex_unlock(&priv->buffer_mutex);
}
@@ -74,7 +74,6 @@ static int tpm_open(struct inode *inode,
}

priv->chip = chip;
- atomic_set(&priv->data_pending, 0);
mutex_init(&priv->buffer_mutex);
setup_timer(&priv->user_read_timer, user_reader_timeout,
(unsigned long)priv);
@@ -89,28 +88,24 @@ static ssize_t tpm_read(struct file *fil
size_t size, loff_t *off)
{
struct file_priv *priv = file->private_data;
- ssize_t ret_size;
+ ssize_t ret_size = 0;
int rc;

del_singleshot_timer_sync(&priv->user_read_timer);
flush_work(&priv->work);
- ret_size = atomic_read(&priv->data_pending);
- if (ret_size > 0) { /* relay data */
- ssize_t orig_ret_size = ret_size;
- if (size < ret_size)
- ret_size = size;
+ mutex_lock(&priv->buffer_mutex);

- mutex_lock(&priv->buffer_mutex);
+ if (priv->data_pending) {
+ ret_size = min_t(ssize_t, size, priv->data_pending);
rc = copy_to_user(buf, priv->data_buffer, ret_size);
- memset(priv->data_buffer, 0, orig_ret_size);
+ memset(priv->data_buffer, 0, priv->data_pending);
if (rc)
ret_size = -EFAULT;

- mutex_unlock(&priv->buffer_mutex);
+ priv->data_pending = 0;
}

- atomic_set(&priv->data_pending, 0);
-
+ mutex_unlock(&priv->buffer_mutex);
return ret_size;
}

@@ -121,17 +116,19 @@ static ssize_t tpm_write(struct file *fi
size_t in_size = size;
ssize_t out_size;

+ if (in_size > TPM_BUFSIZE)
+ return -E2BIG;
+
+ mutex_lock(&priv->buffer_mutex);
+
/* cannot perform a write until the read has cleared
either via tpm_read or a user_read_timer timeout.
This also prevents splitted buffered writes from blocking here.
*/
- if (atomic_read(&priv->data_pending) != 0)
+ if (priv->data_pending != 0) {
+ mutex_unlock(&priv->buffer_mutex);
return -EBUSY;
-
- if (in_size > TPM_BUFSIZE)
- return -E2BIG;
-
- mutex_lock(&priv->buffer_mutex);
+ }

if (copy_from_user
(priv->data_buffer, (void __user *) buf, in_size)) {
@@ -153,7 +150,7 @@ static ssize_t tpm_write(struct file *fi
return out_size;
}

- atomic_set(&priv->data_pending, out_size);
+ priv->data_pending = out_size;
mutex_unlock(&priv->buffer_mutex);

/* Set a timeout by which the reader must come claim the result */
@@ -172,7 +169,7 @@ static int tpm_release(struct inode *ino
del_singleshot_timer_sync(&priv->user_read_timer);
flush_work(&priv->work);
file->private_data = NULL;
- atomic_set(&priv->data_pending, 0);
+ priv->data_pending = 0;
clear_bit(0, &priv->chip->is_open);
put_device(priv->chip->dev);
kfree(priv);