linux-kernel - Re: [PATCH 2/3] TPM: Close data_pending and data

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Date:	Thu, 22 Dec 2011 15:42:55 -0200
From:	Rajiv Andrade <srajiv@...ux.vnet.ibm.com>
To:	Tim Gardner <tim.gardner@...onical.com>
CC:	linux-kernel@...r.kernel.org,
	Seth Forshee <seth.forshee@...onical.com>,
	Debora Velarde <debora@...ux.vnet.ibm.com>,
	Marcel Selhorst <m.selhorst@...rix.com>,
	tpmdd-devel@...ts.sourceforge.net
Subject: Re: [PATCH 2/3] TPM: Close data_pending and data_buffer races

On 20-12-2011 17:39, Tim Gardner wrote:
> On 12/20/2011 09:38 AM, Rajiv Andrade wrote:
>> On 06/12/11 16:29, Tim Gardner wrote:
>>> There is a race betwen tpm_read() and tpm_write where both
>>> chip->data_pending
>>> and chip->data_buffer can be changed by tpm_write() when tpm_read()
>>> clears chip->data_pending, but before tpm_read() grabs the mutex.
>>>
>>> Protect changes to chip->data_pending and chip->data_buffer by 
>>> expanding
>>> the scope of chip->buffer_mutex.
>>>
>>> Reported-by: Seth Forshee<seth.forshee@...onical.com>
>>> Cc: Debora Velarde<debora@...ux.vnet.ibm.com>
>>> Cc: Rajiv Andrade<srajiv@...ux.vnet.ibm.com>
>>> Cc: Marcel Selhorst<m.selhorst@...rix.com>
>>> Cc: tpmdd-devel@...ts.sourceforge.net
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Tim Gardner<tim.gardner@...onical.com>
>>> ---
>>> drivers/char/tpm/tpm.c | 17 +++++++++--------
>>> 1 files changed, 9 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
>>> index b366b34..70bf9e5 100644
>>> --- a/drivers/char/tpm/tpm.c
>>> +++ b/drivers/char/tpm/tpm.c
>>> @@ -1074,12 +1074,15 @@ ssize_t tpm_write(struct file *file, const
>>> char __user *buf,
>>> struct tpm_chip *chip = file->private_data;
>>> size_t in_size = size, out_size;
>>>
>>> + mutex_lock(&chip->buffer_mutex);
>>> +
>>> /* cannot perform a write until the read has cleared
>>> either via tpm_read or a user_read_timer timeout */
>>> - while (atomic_read(&chip->data_pending) != 0)
>>> + while (atomic_read(&chip->data_pending) != 0) {
>>> + mutex_unlock(&chip->buffer_mutex);
>>> msleep(TPM_TIMEOUT);
>>> -
>>> - mutex_lock(&chip->buffer_mutex);
>>> + mutex_lock(&chip->buffer_mutex);
>>> + }
>>>
>>> if (in_size> TPM_BUFSIZE)
>>> in_size = TPM_BUFSIZE;
>>> @@ -1112,22 +1115,20 @@ ssize_t tpm_read(struct file *file, char
>>> __user *buf,
>>>
>>> del_singleshot_timer_sync(&chip->user_read_timer);
>>> flush_work_sync(&chip->work);
>>> - ret_size = atomic_read(&chip->data_pending);
>>> - atomic_set(&chip->data_pending, 0);
>>> + mutex_lock(&chip->buffer_mutex);
>>> + ret_size = atomic_xchg(&chip->data_pending, 0);
>>> if (ret_size> 0) { /* relay data */
>>> ssize_t orig_ret_size = ret_size;
>>> if (size< ret_size)
>>> ret_size = size;
>>>
>>> - mutex_lock(&chip->buffer_mutex);
>>> rc = copy_to_user(buf, chip->data_buffer, ret_size);
>>> memset(chip->data_buffer, 0, orig_ret_size);
>>> if (rc)
>>> ret_size = -EFAULT;
>>
>> What about just moving atomic_set(&chip->data_pending, 0); to here?
>> If I'm not missing anything, this would be cleaner.
>>
>> Rajiv
>
> I'm not sure I agree. Moving just that statement doesn't close the 
> race. Perhaps you could send me your version of this patch so that its 
> clear what you are suggesting.
>
> rtg
diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
index 6a8771f..6a37212b 100644
--- a/drivers/char/tpm/tpm.c
+++ b/drivers/char/tpm/tpm.c
@@ -1210,7 +1210,6 @@ ssize_t tpm_read(struct file *file, char __user *buf,
         del_singleshot_timer_sync(&chip->user_read_timer);
         flush_work_sync(&chip->work);
         ret_size = atomic_read(&chip->data_pending);
-       atomic_set(&chip->data_pending, 0);
         if (ret_size>  0) {     /* relay data */
                 if (size<  ret_size)
                         ret_size = size;
@@ -1223,6 +1222,7 @@ ssize_t tpm_read(struct file *file, char __user *buf,
                 mutex_unlock(&chip->buffer_mutex);
         }  
+       atomic_set(&chip->data_pending, 0);
         return ret_size;
  }

If we reset chip->data_pending after the buffer was copied to userspace,
it's guaranteed that tpm_write() won't touch such buffer before tpm_read()
handles it, given it polls chip->data_pending first.

Rajiv


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