Re: [bug report] HID: ft260: add usb hid to i2c host bridge driver

[Dan Carpenter <dan.carpenter@oracle.com>]

On Sat, Apr 10, 2021 at 03:27:29PM +0300, Michael Zaidman wrote:
> On Fri, Apr 09, 2021 at 03:32:06PM +0300, Dan Carpenter wrote:
> > Hello Michael Zaidman,
> > 
> > The patch 6a82582d9fa4: "HID: ft260: add usb hid to i2c host bridge
> > driver" from Feb 19, 2021, leads to the following static checker
> > warning:
> > 
> > 	drivers/hid/hid-ft260.c:441 ft260_smbus_write()
> > 	error: '__memcpy()' '&rep->data[1]' too small (59 vs 255)
> > 
> > drivers/hid/hid-ft260.c
> >    423  static int ft260_smbus_write(struct ft260_device *dev, u8 addr, u8 cmd,
> >    424                               u8 *data, u8 data_len, u8 flag)
> >    425  {
> >    426          int ret = 0;
> >    427          int len = 4;
> >    428  
> >    429          struct ft260_i2c_write_request_report *rep =
> >    430                  (struct ft260_i2c_write_request_report *)dev->write_buf;
> >    431  
> >    432          rep->address = addr;
> >    433          rep->data[0] = cmd;
> >    434          rep->length = data_len + 1;
> >    435          rep->flag = flag;
> >    436          len += rep->length;
> >    437  
> >    438          rep->report = FT260_I2C_DATA_REPORT_ID(len);
> >    439  
> >    440          if (data_len > 0)
> >    441                  memcpy(&rep->data[1], data, data_len);
> >                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > Smatch says that this can be called from the i2cdev_ioctl_smbus()
> > function.
> 
> Hi Dan,
> 
> This is an example of a false-positive static checker warning.
> 
> The maximum data size that the i2cdev_ioctl_smbus() can pass to the
> i2c_smbus_xfer() is sizeof(data->block) which is (I2C_SMBUS_BLOCK_MAX + 2)
> or 34 bytes. Thus, no need to check the data_len against 59 here.
> 
> > 
> > i2cdev_ioctl_smbus()
> >   --> i2c_smbus_xfer
> >       --> __i2c_smbus_xfer
> >           --> ft260_smbus_xfer
> >               --> ft260_smbus_write

It's actually me who misunderstood the Smatch warning.  Smatch is not
complaining about data_len, it's data->block[0] which is user
controlled and only for the I2C_SMBUS_I2C_BLOCK_DATA command.

The call tree is the same.  I've looked at it again.  Here is how
i2cdev_ioctl_smbus() looks like:

drivers/i2c/i2c-dev.c
   355                  return -EINVAL;
   356          }
   357  
   358          if ((size == I2C_SMBUS_BYTE_DATA) ||
   359              (size == I2C_SMBUS_BYTE))
   360                  datasize = sizeof(data->byte);
   361          else if ((size == I2C_SMBUS_WORD_DATA) ||
   362                   (size == I2C_SMBUS_PROC_CALL))
   363                  datasize = sizeof(data->word);
   364          else /* size == smbus block, i2c block, or block proc. call */
   365                  datasize = sizeof(data->block);
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   366  
   367          if ((size == I2C_SMBUS_PROC_CALL) ||
   368              (size == I2C_SMBUS_BLOCK_PROC_CALL) ||
   369              (size == I2C_SMBUS_I2C_BLOCK_DATA) ||
                             ^^^^^^^^^^^^^^^^^^^^^^^^
   370              (read_write == I2C_SMBUS_WRITE)) {
   371                  if (copy_from_user(&temp, data, datasize))
                                            ^^^^
temp.block[0] is user controlled.

   372                          return -EFAULT;
   373          }
   374          if (size == I2C_SMBUS_I2C_BLOCK_BROKEN) {
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   375                  /* Convert old I2C block commands to the new
   376                     convention. This preserves binary compatibility. */
   377                  size = I2C_SMBUS_I2C_BLOCK_DATA;
   378                  if (read_write == I2C_SMBUS_READ)
   379                          temp.block[0] = I2C_SMBUS_BLOCK_MAX;
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Except for size BROKEN

   380          }
   381          res = i2c_smbus_xfer(client->adapter, client->addr, client->flags,
   382                read_write, command, size, &temp);
                                                 ^^^^^

   383          if (!res && ((size == I2C_SMBUS_PROC_CALL) ||
   384                       (size == I2C_SMBUS_BLOCK_PROC_CALL) ||
   385                       (read_write == I2C_SMBUS_READ))) {
   386                  if (copy_to_user(data, &temp, datasize))
   387                          return -EFAULT;
   388          }

The rest of the call tree seems straight forward but it's possible I
have missed somewhere that checks data[0].  Here is how ft260_smbus_xfer()
looks like.

drivers/hid/hid-ft260.c
   655          case I2C_SMBUS_BLOCK_DATA:
   656                  if (read_write == I2C_SMBUS_READ) {
   657                          ret = ft260_smbus_write(dev, addr, cmd, NULL, 0,
   658                                                  FT260_FLAG_START);
   659                          if (ret)
   660                                  goto smbus_exit;
   661  
   662                          ret = ft260_i2c_read(dev, addr, data->block,
   663                                               data->block[0] + 1,
   664                                               FT260_FLAG_START_STOP_REPEATED);
   665                  } else {
   666                          ret = ft260_smbus_write(dev, addr, cmd, data->block,
   667                                                  data->block[0] + 1,
   668                                                  FT260_FLAG_START_STOP);
   669                  }
   670                  break;
   671          case I2C_SMBUS_I2C_BLOCK_DATA:
   672                  if (read_write == I2C_SMBUS_READ) {
   673                          ret = ft260_smbus_write(dev, addr, cmd, NULL, 0,
   674                                                  FT260_FLAG_START);
   675                          if (ret)
   676                                  goto smbus_exit;
   677  
   678                          ret = ft260_i2c_read(dev, addr, data->block + 1,
   679                                               data->block[0],
   680                                               FT260_FLAG_START_STOP_REPEATED);
   681                  } else {
   682                          ret = ft260_smbus_write(dev, addr, cmd, data->block + 1,
   683                                                  data->block[0],
                                                        ^^^^^^^^^^^^^^
Boom.  Dead.

   684                                                  FT260_FLAG_START_STOP);
   685                  }
   686                  break;
   687          default:
   688                  hid_err(hdev, "unsupported smbus transaction size %d\n", size);
   689                  ret = -EOPNOTSUPP;
   690          }

regards,
dan carpenter