This page contains a technical description of the back-door found in Samsung Galaxy devices.
For a general description of the issue, please refer to the statement published on the Free Software Foundation's website.
This back-door is present in most proprietary Android systems running on the affected Samsung Galaxy devices, including the ones that are shipped with the devices. However, when Replicant is installed on the device, this back-door is not effective: Replicant does not cooperate with back-doors.
Abstract
Samsung Galaxy devices running proprietary Android versions come with a back-door that provides remote access to the data stored on the device.
In particular, the proprietary software that is in charge of handling the communications with the modem, using the Samsung IPC protocol, implements a class of requests known as RFS commands, that allows the modem to perform remote I/O operations on the phone's storage. As the modem is running proprietary software, it is likely that it offers over-the-air remote control, that could then be used to issue the incriminated RFS messages and access the phone's file system.
Known affected devices
The following table shows which devices are known to contain this back-door as part of the software they ship with.
Please contact us if you know about some other device that could be concerned by this back-door or have more information on one of the listed devices!
| Device | Incriminated program running as root | SELinux enabled | libsamsung-ipc support | Replicant support |
|---|---|---|---|---|
| Nexus S (I902x) | No | Possible with Android 4.2 and later | Yes | Yes |
| Galaxy S (I9000) | Yes | ? | Yes | Yes |
| Galaxy S 2 (I9100) | No | ? | Yes | Yes |
| Galaxy Note (N7000) | No | ? | Yes | Yes |
| Galaxy Nexus (I9250) | No | Possible with Android 4.2 and later | Yes | Yes |
| Galaxy Tab 2 7.0 (P31xx) | No | ? | Yes | Yes |
| Galaxy Tab 2 10.1 (P51xx) | No | ? | Yes | Yes |
| Galaxy S 3 (I9300) | No | ? | Yes | Yes |
| Galaxy Note 2 (N7100) | No | ? | Yes | Yes |
Back-door sample
In order to investigate the back-door and check what it actually lets the modem do, some code was added to the modem kernel driver to make it craft and inject requests using the incriminated messages and check its results.
The following patch: 0001-modem_if-Inject-and-intercept-RFS-I-O-messages-to-pe.patch (to apply to the SMDK4412 Replicant 4.2 kernel) implements a sample use of the back-door that will:- open the
/data/radio/testfile - read its content
- close the file
This demonstrates that the incriminated software will execute these operations upon modem request. Note that the software implementation appends /efs/root/ to the provided path, but it's fairly simple to escape that path and request any file on the file system (using ../../). Note that the files are opened with the incriminated software's user permissions, which may be root on some devices. On other cases, its runs as an unprivileged user that can still access the user's personal data (/sdcard). Finally, some devices may implement SELinux, which considerably restricts the scope of possible files that the modem can access, including the user's personal data (/sdcard/).
The following sample was obtained on a Galaxy Note 2 (N7100) running CyanogenMod 10.1.3.
Sample file
The sample file used for this demonstration (/data/radio/test) is filled with "Hello World!":
Kernel log
The relevant part is the response to the read request:
which matches the content of the
/data/radio/test file, hence making it obvious that the incriminated software implements the back-door.Incriminated software log
Analysis
The following analysis was conducted using the libsec-ril.so binary file (the incriminated proprietary software) as extracted from the CyanogenMod 10.1.3 system zip for the Galaxy S 3 (I9300), from location system/lib/libsec-ril.so.
The developers involved in the present analysis did not ever agree to any sort of End User License Agreement that explicitly prohibited the reverse engineering and decompiling operations of the incriminated binary. The reverse engineering operations that led to these findings originally took place during the development of Samsung-RIL, the free software replacement for the incriminated program. Hence, we believe these operations were conducted for the sole purpose of interoperability and not with the intent of creating a competing product. As the involved developers were based in Europe, we believe the legality of these operations is granted by article 6 of the 1991 EU Computer Programs Directive.
As a first approach, using the strings tool against the incriminated program reveals numerous suspicious command names that appear to be Samsung IPC protocol definitions:
The names of these commands make it obvious that they let the modem perform I/O operations.
The strings utility also reveals matching function names that seem to implement the handling of these commands:
Taking a closer look at these functions, using the objdump decompiler, reveals that they are actually called from the ipc_recv_rfs function, itself called fromprocess_ipc_notify_message, which appears to handle the received messages from the modem. Hence we can deduct that the incriminated functions are actually called upon modem request.
Taking a closer look at one of these functions, e.g. RxRFS_ReadFile reveals multiple calls to the Procedure Linkage Table (PLT). Hence we believe these calls are linked functions from the libc library, especially I/O-related functions such as (in a general manner) open, close, read, write, etc.
Samsung IPC RFS messages
The following table associates each Samsung IPC RFS message with its hexadecimal command value:
| Message | Hexadecimal command value |
|---|---|
| IPC_RFS_NV_READ_ITEM | 0x01 |
| IPC_RFS_NV_WRITE_ITEM | 0x02 |
| IPC_RFS_READ_FILE | 0x03 |
| IPC_RFS_WRITE_FILE | 0x04 |
| IPC_RFS_LSEEK_FILE | 0x05 |
| IPC_RFS_CLOSE_FILE | 0x06 |
| IPC_RFS_PUT_FILE | 0x07 |
| IPC_RFS_GET_FILE | 0x08 |
| IPC_RFS_RENAME_FILE | 0x09 |
| IPC_RFS_GET_FILE_INFO | 0x0a |
| IPC_RFS_UNLINK_FILE | 0x0b |
| IPC_RFS_MAKE_DIR | 0x0c |
| IPC_RFS_REMOVE_DIR | 0x0d |
| IPC_RFS_OPEN_DIR | 0x0e |
| IPC_RFS_READ_DIR | 0x0f |
| IPC_RFS_CLOSE_DIR | 0x10 |
| IPC_RFS_OPEN_FILE | 0x11 |
| IPC_RFS_FTRUNCATE_FILE | 0x12 |
| IPC_RFS_GET_HANDLE_INFO | 0x13 |
| IPC_RFS_CREATE_FILE | 0x14 |
| IPC_RFS_NV_WRITE_ALL_ITEM | 0x15 |
Legitimacy
The incriminated RFS messages of the Samsung IPC protocol were not found to have any particular legitimacy nor relevant use-case. However, it is possible that these were added for legitimate purposes, without the intent of doing harm by providing a back-door. Nevertheless, the result is the same and it allows the modem to access the phone's storage.
However, some RFS messages of the Samsung IPC protocol are legitimate (IPC_RFS_NV_READ_ITEM and IPC_RFS_NV_WRITE_ITEM) as they target a very precise file, known as the modem's NV data. There should be no particular security concern about these as both the proprietary implementation and its free software replacement strictly limit actions to that particular file.
Areas of work
Some work could be done in order to handle that back-door:- Samsung-RIL could show a message alerting the user when the back-door is being used, including the requested path and asking the user to save logs and contact us.
- Alternatively, the kernel could block the incriminated RFS requests and keep a trace of them in the logs for the record. That option would work for CyanogenMod, where the incriminated proprietary blob is still used.
Notes
Our free software replacement for the incriminated binary is Samsung-RIL which relies on libsamsung-ipc: both are used in Replicant.
The affected devices have modems that use the Samsung IPC protocol, mostly Intel XMM6160 and Intel XMM6260 modems. Note that despite this back-door, the devices using these modems are most likely to have good modem isolation, compared to other devices using Qualcomm platforms. Bear in mind that this back-door is implemented in software and can easily be removed by installing a free replacement for the incriminated software, for instance by installing Replicant. Hence, we don't consider the incriminated devices to be inherently bad targets because of this back-door.
Comments
Post a Comment