Messages in this thread | | | Date | Thu, 22 Dec 2011 11:44:10 -0700 | From | Tim Gardner <> | Subject | Re: [PATCH 2/3] TPM: Close data_pending and data_buffer races |
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On 12/22/2011 10:42 AM, Rajiv Andrade wrote: > 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@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 > 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. >
NAK - this patch makes it worse (if I'm reading your email client garbled patch correctly). Now it races with tpm_write() as well as timeout_work(). You cannot futz with chip->data_pending outside of the exclusion zones. Consider what will happen if a user process just loops doing reads. chip->data_pending gets cleared every time tpm_read() is called, regardless of what tpm_write() or timeout_work() are doing at the time.
tpm_read() / tpm_write() is a simple producer consumer model. Just use mutexes in an uncomplicated way. There is no need for data_pending to be atomic_t.
rtg -- Tim Gardner tim.gardner@canonical.com
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