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

From: Rajiv Andrade
Date: Thu Dec 22 2011 - 15:02:25 EST


Thanks, Rajiv Andrade Security Development IBM Linux Technology Center

On 22-12-2011 16:44, Tim Gardner wrote:
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@xxxxxxxxxxxxx>
Cc: Debora Velarde<debora@xxxxxxxxxxxxxxxxxx>
Cc: Rajiv Andrade<srajiv@xxxxxxxxxxxxxxxxxx>
Cc: Marcel Selhorst<m.selhorst@xxxxxxxxxx>
Cc: tpmdd-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tim Gardner<tim.gardner@xxxxxxxxxxxxx>
---
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.

Not sure how it's displaying for you, but your mail client is eating all whitespaces when sending. Look back here what I said:

http://marc.info/?l=tpmdd-devel&m=132439922903276&w=2

It's inside the mutex region.

This would require another fix though. tpm_write() doesn't check tpm_transmit return code (and it should).
In case it returns an error (< 0), chip->data_pending would remain the same forever with that change.


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

That's a separate patch, 3/3, which is good once chip->data_pending is handled inside such regions.

Rajiv

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