Acer Iconia A500

This is an attempt to summarize and extend the current knowledge about the Acer Iconia A500 tablet.

TOC

Hardware

Tegra reserved memory:
LP0:1840c0001840dfff
Bootloader framebuffer: 00000000ffffffff
Framebuffer:2e7000002eefffff
2nd Framebuffer:2ef000002fefffff
Carveout:2ff000003fefffff

Kernel command line: nvmem=128M@384M mem=1024M@0M vmalloc=256M video=tegrafb console=none usbcore.old_scheme_first=1 lp0_vec=8192@0x1840c000 tegraboot=sdmmc gpt

Partition layout

The parition layout is in a NVIDIA proprietary format with no public official specification. It have however been reverse engineered based on the Toshiba AC100/Dynabook AZ: Patch nvtegra.c and here.

IDNameTypeSizeImageDeviceMountpoint
2BCTboot_config_table (basic)3145728 (3M)
3PTpartition_table (basic)4096*128 (4K data repeated)
4EBTbootloader (basic)4194304 (4M)bootloader.bin
5GP1GP1 (basic)1048576 (1M)
6SOSdata (basic)5242880 (5M)recovery.imgmmcblk0p1
7LNXdata (basic)8388608 (8M)boot.imgmmcblk0p2
8APPdata (ext4)629145600 (600M)system.imgmmcblk0p3/system
9CACdata (ext3)1283457024 (1224M)mmcblk0p4/cache
10MSCdata (ext3)4194304 (4M)mmcblk0p5
11FLXdata (ext4)104857600 (100M)flexrom.imgmmcblk0p6/system/vendor
12AKBdata (basic)10485760 (10M)mmcblk0p7
13UDAdata (ext3)29955719168 (28568M)mmcblk0p8/data
14GPTGPT (basic)0xFFFFFFFFFFFFFFFF

/mnt/sdcard is fuse mounted from /data/media.

BCT and EBT are encrypted using the SBK.

Boot Config Table (BCT)

The Boot Config Table (BCT) contain quite a bit of information. There is a bct_dump tool available trough the chromium project, the source can be found in the cbootimage repository.

File layout

OffsetSizeDescription
0x00x10Checksum - AES-CMAC of the rest of the file
0x100x10All zero?
0x200x4Version (0x20001)
0x240x4Block size in log2 (14 => 2**14 = 0x4000)
0x280x4Page size in log2 (9 => 2**9 = 0x200)
0x2C0x4Partition size (0x1000000)
0x300x4Number of parameter sets (0x4)
0x340x4Device 1 type
0: None
1: NAND
3: SPI
4: SDMMC
5: MAX
0x380x4Device 2 type
0x3C0x4Device 3 type
0x400x4Device 4 type
0x440x10Device 1 params
0x540x10Device 2 params
0x640x10Device 3 params
0x740x10Device 4 params
0x840x4Number of SDRAM sets (0x4)
0x880x200SDROM set 1?
0x2880x200SDROM set 2?
0x4880x200SDROM set 3?
0x6880x200SDROM set 4?
0x8880x?Bad block table?
0xA900x4Active Boot loader
Boot loader 1
0xA940x4Boot loader version
0xA980x4Boot loader start block
0xA9C0x4Boot loader start page
0xAA00x4Boot loader length
0xAA40x4Boot loader load address
0xAA80x4Boot loader entry point
0xAAC0x4Boot loader attribute
0xAB00x10Boot loader hash
Boot loader 2
Boot loader 3
Boot loader 4
0xB440x4A8Customer Data
0xFEC0x1Enable Fail Back
0xFED0x3Reserved (padding)

ODM Data

ODM Data is a 32bit value that can be found at offset 0xfe4 in the BCT.

The bitfield values for ODM Data is specified in tegra_devkit_custopt.h.

BitsUseValues
0-7PERSONALITY...
8-10TRANSPORT0 = DEFAULT
1 = NONE
2 = UART
3 = USB
4 = ETHERNET
11-12UART0 = DEFAULT
1 = A
2 = B
3 = C
13-14ETHERNET0 = DEFAULT
1 = SPI
15-17CONSOLE OPTION0 = UARTA (DEFAULT)
1 = UARTB
...
4 = UARTE
18-19CONSOLE0 = DEFAULT
1 = NONE
2 = DCC
3 = UART
20-22DISPLAY0 = EMBEDDED (DEFAULT)
1 = HDMI
3 = CRT
23DHCP0 = DEFAULT
1 = ENABLED
24-27?...
28-30MEMORY0 = 512MB
1 = 256MB
2 = 512MB
3 = 1024MB
31LPSTATE0 = LP0
1 = LP1

So with ODM 0x300d8011 = 0 011 0000 0 000 11 011 00 00 000 00010001 we get

0123 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
PERSONALITY TRANSPORT UART ETH CONSOLE CONSOLE DISPLAY DHCP ? MEMORY LPSTATE
10001000 000 00 00 110 11 000 0 0000 110 0
VOYAGER, eMMC, with TRACE DEFAULT DEFAULT DEFAULT UARTD UART EMBEDDED DEFAULT ? 1024M LP0

Partition Table (PT)

The custom Tegra partition table.

Booloader (EBT)

Boot verification... checking checksums, signing?

Boot Partitons (LNX and SOS)

The boot partitons LNX and SOS contain combined kernel and initrd images, following the format described in bootimg.h

AKB

Checksums for the rest of the partitions, and what more?

Seems to contain Widevine DRM key storage?

To trigger a rebuild of the checksums on boot replace 64 bytes at offset 0x84 with "00 FB 30 94 99 01 4F 97 2E 4C 2B A5 18 6B DD 06" x 4.

Flashing

When in APX mode the device can be flashed using NVIDIA's nvflash.

Secure Boot Key (SBK)

A description of what Secure Boot Key (SBK) is can be found at http://androidroot.mobi/technical/tf-secure-boot-key/.

The SBK used for the Iconia A500 is generated based on the UID of the device and thus unique for each device.

Getting the UID

From a working device

The UID is also the USB serial numer of the device.

The serial can be extracted from HAL:

hal-get-property --udi "$(hal-find-by-property --key 'info.product' --string 'Iconia tablet A500')" --key 'usb_device.serial'

From APX mode

The device will respond with the UID to the first read after it have been started in APX. See apxuid.

Generating the SBK

For clarification, as there seem to be some confusion, this describes a vendor/model specific algorithm that is used by a few Acer models.

To generate the SBK from the UID (assumign UID is a hexadecimal string)

  1. Discard any leading 0x in the UID
  2. Split the UID into four 4 character strings
  3. For each part, take the ascii values and multiply with 100 raised to the position.
    e.g. "89AB" => 56*100**3 + 57*100**2 + 65*100**2 + 66*100**0 = 56576566.
  4. xor
  5. If using a little-endian architecture, swap the byte order
  6. Print the key

A C implementation of the algorithm for generating the key can be found here.

Decrypting data encrypted with the SBK

The SBK is used to encrypt BCT and EBT AES128 CBC. A small tool to decrypt (libgcrypt based) can be found here.

APX Mode

APX mode is the NVIDIA Tegra recovery mode, when in APX mode the device will identify as a NVIDIA device 0955:7820. There are a few different ways to get the A500 into APX mode.

It might also be a good idea to create a udev rule to expand write access to the APX device.

cat /etc/udev/rules.d/51-android.rules

SUBSYSTEM=="usb", ATTR{idVendor}=="0502",ATTR{idProduct}=="3325", MODE="0666", SYMLINK+="a500" SUBSYSTEM=="usb", ATTR{idVendor}=="0955",ATTR{idProduct}=="7820", MODE="0666"

Acquiring nvflash

Nvflash is part of the Linux for Tegra package available from Tegra 250 & Tango Development Kits.

dmesg on the Iconia A500 says something about ventana, can we use the ventana fastboot.bin? The profile in the ODM Data points in another direction?

Extracting BCT

In here lies a dilemma, to extract the BCT using nvflash we need the BCT.

As root we can use dd from mmcblk0 and then decryp using the SBK, if we don't have root our only choice seems to be to find a matching BCT somewhere else...

$ cat /data/local/tmp/test.sh

#!/system/bin/sh echo test /system/bin/dd if=/dev/block/mmcblk0 bs=512 count=13312 of=/sdcard/mmcblk0_start

$ cmdclient ec_micswitch \`/data/local/tmp/test.sh\`

Then we can extract the bct from the dump

./sbkdecrypt --sbk 0x00000000 0x00000000 0x00000000 0x00000000 mmcblk0_start a500.bct 0x0 0xFF0

The first 16 bytes are not actually part of the encrypted data, they are a AES-CMAC checksum for the other 4064 bytes, so decrypting like this will actually not get the correct result for the first 32 bytes (as we are using CBC we mess up the decryption of the first block by decrypting the block before it). This does not really seem to matter in how it is used though.

And the odm data:

hexdump -n 4 -s 0xfe4 -e '1/4 "0x%08x\n"' a500.bct

0x300d8011

Extracting bootloader

The bootloader can also be extracted from the dump

./sbkdecrypt --sbk 0x00000000 0x00000000 0x00000000 0x00000000 mmcblk0_start bootloader.bin 0x280000 0xAE691

Using nvflash

./nvflash --bl acer.bin --sbk 0x00000000 0x00000000 0x00000000 0x00000000 --getpartitiontable test.txt

Fails due to "Bct file not found"

--getpartitiontable partitiontable.txt

./nvflash --bl bootloader.bin --getbct --bct backup.bct --go ./nvflash -r --getpartitiontable partition.txt ./nvflash -r --read 2 part2.img

The APX Protocol

Undocumented. Could perhaps be a fun project to dump using wireshark and try to create an open alternative to nvflash.

Parts

Tracing a --getpartitiontable:

  1. Get UID
    1. Host does USB bulk read - Device returns USB serial/CPUID
  2. Get RCM version
    1. Host does USB bulk write of 1028 bytes - request for rcm version (?)
      First 4B is Length, followed by a 16B hash, followed by the payload with some data inserted into.
      In this case there is no payload so most of the data is zeros.
    2. Host does USB bulk read - Device returns "01 00 02 00" = 0x20001 = rcm version
  3. Upload miniloader
    1. Host does bulk writes, total of 4096*13 + 2980 = 56228 bytes
      Same format as previous message but with the miniloader as payload, sending some MiniLoader, an image that is built info nvflash.
    2. Host does USB bulk read - Device returns "00 00 00 00"
  4. Switch to using Nv3p messages
  5. Get System information
    1. Write 24B - Request for system information?
    2. Read 16B - Ack on the request
    3. Read 16B - system information? or just ack for the message?
    4. Read 48B - Starts with CPUID must be system information
    5. Read 4B - checksum for system information
    6. Write 16B - Ack
  6. ...

Rooting

Android 3.1 (Acer_A500_4.0.10.42_COM_GEN1) kernel 2.6.36.3

cmdclient can be exploited to run commands as root. For example cmdclient ec_micswitch \`mount -o remount,rw /system\`.

Cross compiling

Native binaries are always fun to have...

Acquiring a toolchain

There are a few choices on which toolchain to use, it seems using the one provided through the Android project may be easiest.

git clone git://github.com/android/platform_prebuilt.git

Building

blah blah

adb push <file> /data/local/tmp

Various

Mixed stuff that could be usefull.

USB Capture with Wireshark
Some info on the ADB protocol

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Uptime: 219 days, 16 hours, 24 minutes