Vulnerabilities > CVE-2017-0412 - Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability in Google Android 7.0/7.1.0/7.1.1
Attack vector
LOCAL Attack complexity
LOW Privileges required
NONE Confidentiality impact
HIGH Integrity impact
HIGH Availability impact
HIGH Summary
An elevation of privilege vulnerability in the Framework APIs could enable a local malicious application to execute arbitrary code within the context of a privileged process. This issue is rated as High because it could be used to gain local access to elevated capabilities, which are not normally accessible to a third-party application. Product: Android. Versions: 7.0, 7.1.1. Android ID: A-33039926.
Vulnerable Configurations
| Part | Description | Count |
|---|---|---|
| OS | 3 |
Common Weakness Enumeration (CWE)
Common Attack Pattern Enumeration and Classification (CAPEC)
- Leveraging Race Conditions via Symbolic Links This attack leverages the use of symbolic links (Symlinks) in order to write to sensitive files. An attacker can create a Symlink link to a target file not otherwise accessible to her. When the privileged program tries to create a temporary file with the same name as the Symlink link, it will actually write to the target file pointed to by the attackers' Symlink link. If the attacker can insert malicious content in the temporary file she will be writing to the sensitive file by using the Symlink. The race occurs because the system checks if the temporary file exists, then creates the file. The attacker would typically create the Symlink during the interval between the check and the creation of the temporary file.
- Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions This attack targets a race condition occurring between the time of check (state) for a resource and the time of use of a resource. The typical example is the file access. The attacker can leverage a file access race condition by "running the race", meaning that he would modify the resource between the first time the target program accesses the file and the time the target program uses the file. During that period of time, the attacker could do something such as replace the file and cause an escalation of privilege.
Exploit-Db
file exploits/android/dos/41355.txt id EDB-ID:41355 last seen 2018-11-30 modified 2017-02-14 platform android port published 2017-02-14 reporter Exploit-DB source https://www.exploit-db.com/download/41355 title Google Android - android.util.MemoryIntArray Ashmem Race Conditions type dos description Android - Inter-Process munmap due to Race Condition in ashmem. CVE-2017-13216. Dos exploit for Android platform file exploits/android/dos/43464.txt id EDB-ID:43464 last seen 2018-01-24 modified 2018-01-08 platform android port published 2018-01-08 reporter Exploit-DB source https://www.exploit-db.com/download/43464/ title Android - Inter-Process munmap due to Race Condition in ashmem type dos
Seebug
| bulletinFamily | exploit |
| description | The MemoryIntArray class allows processes to share an in-memory array of integers by transferring an ashmem file descriptor. As the class implements the Parcelable interface, it can be passed within a Parcel or a Bundle and transferred via binder to remote processes. Instead of directly tracking the size of the shared memory region, the MemoryIntArray class calls the ASHMEM_GET_SIZE ioctl on the ashmem descriptor to retrieve it on demand. This opens up a variety of race conditions when using MemoryIntArray, as the size of the ashmem descriptor can be modified (via ASHMEM_SET_SIZE) so long as the descriptor itself has not yet been mapped. To illustrate this, here is a snippet from the native function called when a MemoryIntArray is first mapped in: ``` 1. static jlong android_util_MemoryIntArray_open(JNIEnv* env, jobject clazz, jint fd, 2. jboolean owner, jboolean writable) 3. { 4. if (fd < 0) { 5. jniThrowException(env, "java/io/IOException", "bad file descriptor"); 6. return -1; 7. } 8. 9. int ashmemSize = ashmem_get_size_region(fd); 10. if (ashmemSize <= 0) { 11. jniThrowException(env, "java/io/IOException", "bad ashmem size"); 12. return -1; 13. } 14. 15. int protMode = (owner || writable) ? (PROT_READ | PROT_WRITE) : PROT_READ; 16. void* ashmemAddr = mmap(NULL, ashmemSize, protMode, MAP_SHARED, fd, 0); 17. ... 18.} ``` If an attacker can call ASHMEM_SET_SIZE on the shared ashmem descriptor during the execution of lines 10-15, he may modify the internal size of the descriptor, causing a mismatch between the mapped-in size and the underlying size of the descriptor. As the MemoryIntArray class uses the size reported by the ashmem descriptor to perform all bounds checks (see http://androidxref.com/7.0.0_r1/xref/frameworks/base/core/java/android/util/MemoryIntArray.java#217), this allows an attacker to cause out-of-bounds accesses to the mapped in buffer via subsequent calls to the "get" and "set" methods. Additionally, MemoryIntArray uses the ashmem-reported size when unmapping the shared memory buffer, like so: ``` 1. static void android_util_MemoryIntArray_close(JNIEnv* env, jobject clazz, jint fd, 2. jlong ashmemAddr, jboolean owner) 3. { 4. ... 5. int ashmemSize = ashmem_get_size_region(fd); 6. if (ashmemSize <= 0) { 7. jniThrowException(env, "java/io/IOException", "bad ashmem size"); 8. return; 9. } 10. int unmapResult = munmap(reinterpret_cast<void *>(ashmemAddr), ashmemSize); 11. ... 12.} ``` This allows an attacker to trigger an inter-process munmap with a controlled size by modifying the underlying ashmem size to a size larger than the mapped in buffer's size. Doing so will cause the finalizer to call munmap with the new size, thus forcibly freeing memory directly after the buffer. After the memory is freed, the attacker can attempt to re-capture it using controlled data. I've attached a PoC which triggers this race condition and causes system_server to call munmap on a large memory region. Running it should cause system_server to crash. Note that simply modifying the size of the ashmem file descriptor is insufficient. This is due to the fact that Parcel objects keep track of the size of the ashmem descriptors passed through them using an unsigned variable (http://androidxref.com/7.0.0_r1/xref/frameworks/native/libs/binder/Parcel.cpp#216). When a descriptor object is released, the size variable is decremented according to the reported size of the descriptor. Although this variable is not used in any meaningful way, increasing the size of the ashmem descriptor between the creation and destruction of a Parcel would cause the size variable to underflow. As system_server is compiled with UBSAN, this triggers an abort (thus preventing us from using the exploit). To get around this, I've added an additional descriptor to the Parcel, whose size is appropriately reduced before increasing the size of the MemoryIntArray's descriptor (thus keeping the size variable from underflowing). |
| id | SSV:92902 |
| last seen | 2017-11-19 |
| modified | 2017-04-05 |
| published | 2017-04-05 |
| reporter | Root |
| title | Android: Ashmem race conditions in android.util.MemoryIntArray (CVE-2017-0412) |
References
- http://www.securityfocus.com/bid/96056
- http://www.securityfocus.com/bid/96056
- http://www.securitytracker.com/id/1037798
- http://www.securitytracker.com/id/1037798
- https://source.android.com/security/bulletin/2017-02-01.html
- https://source.android.com/security/bulletin/2017-02-01.html
- https://www.exploit-db.com/exploits/41355/
- https://www.exploit-db.com/exploits/41355/