Re: [PATCH 2/3] TPM: Close data_pending and data_buffer races

[Tim Gardner <tim.gardner@canonical.com>]

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@canonical.com>
>> Cc: Debora Velarde<debora@linux.vnet.ibm.com>
>> Cc: Rajiv Andrade<srajiv@linux.vnet.ibm.com>
>> Cc: Marcel Selhorst<m.selhorst@sirrix.com>
>> Cc: tpmdd-devel@lists.sourceforge.net
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Tim Gardner<tim.gardner@canonical.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
-- 
Tim Gardner tim.gardner@canonical.com