Android ssp_batch_ioctl Out-Of-Bounds Write

2017.01.17
Credit: laginimaineb
Risk: High
Local: Yes
Remote: No
CVE: N/A
CWE: N/A

Android: OOB write in ssp_batch_ioctl SensorHub exposes a character device under /dev/batch_io which can be used in order to send instructions to batches of running sensors. The IOCTL handler from this device has the following high-level logic: 1. static long ssp_batch_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { 2. struct ssp_data *data = container_of(file->private_data, struct ssp_data, batch_io_device); 3. <...SNIP...> 4. sensor_type = (cmd & 0xFF); 5. if ((cmd >> 8 & 0xFF) != BATCH_IOCTL_MAGIC) { 6. pr_err("[SSP] Invalid BATCH CMD %x", cmd); 7. return -EINVAL; 8. } 9. <...SNIP...> 10. if (ret > 0) { 11. data->batchOptBuf[sensor_type] = (u8)batch.flag; 12. data->batchLatencyBuf[sensor_type] = timeout_ms; 13. data->adDelayBuf[sensor_type] = batch.delay; 14. } else { 15. ret = send_instruction_sync(data, CHANGE_DELAY, sensor_type, uBuf, 9); 16. if (ret > 0) { 17. data->batchOptBuf[sensor_type] = (u8)batch.flag; 18. data->batchLatencyBuf[sensor_type] = timeout_ms; 19. data->adDelayBuf[sensor_type] = batch.delay; 20. } 21. ... As seen in the code above, the "sensor_type" field is taken from the least-significant byte of the user-supplied "cmd" argument. The range of the sensor_type is not validated in any way before using it (lines 11-19) as an index into a number of arrays defined under the "ssp_data" structure. Looking at the definition of the "ssp_data" structure shows that the underlying arrays are defined as follows: struct ssp_data { ... int64_t adDelayBuf[SENSOR_MAX]; u64 lastTimestamp[SENSOR_MAX]; s32 batchLatencyBuf[SENSOR_MAX]; s8 batchOptBuf[SENSOR_MAX]; ... int (*wakeup_mcu)(void); int (*set_mcu_reset)(int); void (*get_sensor_data[SENSOR_MAX])(char *, int *, struct sensor_value *); void (*report_sensor_data[SENSOR_MAX])(struct ssp_data *, struct sensor_value *); ... } The value of SENSOR_MAX varies according to the sensor vendor. For Broadcom, SENSOR_MAX is defined as 30, for Atmel 21, and for STM 25. Regardless, as the user-supplied "sensor_type" argument is not validated in any way, its range is 0x00-0xFF, thus exceeding the dimensions of the arrays above. An attacker may use this vulnerability in order to overwrite values in the "ssp_data" structure which reside after the aforementioned arrays. For example, there are a few function pointers (see above) which the attacker could hijack. Moreover, note that the flow leading to the memory corruption is fully attacker-controlled, as is some of the data written (e.g., batch.flag or batch.delay). This issue can be addressed by verifying the range of the "sensor_type" field to make sure it doesn't exceed SENSOR_MAX. I've statically verified this issue on an SM-G935F device. The open-source kernel package I analysed was "SM-G935F_MM_Opensource". The device entry mentioned above is owned by UID/GID "system". The SELinux context for this device is: "u:object_r:io_device:s0". According to the default SELinux rules as present on the SM-G935F (version XXS1APG3), the following contexts may access this device file: allow system_server io_device : chr_file { ioctl read open } ; This bug is subject to a 90 day disclosure deadline. If 90 days elapse without a broadly available patch, then the bug report will automatically become visible to the public. Found by: laginimaineb


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