LwVM

LwVM (Lightweight Volume Manager) - partition table, which is used by iOS since 5.0.

com.apple.driver.LightweightVolumeManager kext wraps the block device (/dev/disk0) where the actual LwVM partition table is stored and publishes /dev/disk0s1 with emulated GTP which can be edited from user-land using conventional GTP editing tools (e.g. gptfdisk) [1]. All edits are trapped by the kext and before shutting down (after the volumes are unmounted) it writes the corresponding changes to LwVM partition table on /dev/disk0 (implemented in LightweightVolumeManager::_gptLazyRepartition() function from the LwVM kext). After next boot new devices (e.g. /dev/rdisk0s1s3) are published by the kext. This mechanism is used during OTA Updates to create third partition where the update ramdisk is copied to and booted into by setting nvram boot-command to upgrade [2] and probably also boot-partition to 2 (partitions are indexed from 0, so third partition).

The update partition is set up by com.apple.MobileSoftwareUpdate.CleanupPreparePathService (XPC service accompanying softwareupdated). It shrinks HFS of the second (data) partition using fsctl as illustrated in hfs_resize. It then edits GTP on /dev/rdisk0s1s3 to reflect the size of shrunk HFS and creates new entry for the update partition in the freed space and creates new HFS on it. To create file-system manually you can use e.g.: newfs_hfs -s -b 8192 -J 8192k -v Data2 /dev/rdisk0s1s4.

Structure

typedef struct _LwVMPartitionRecord {
        uint64_t type[2]; //Should be equal to LwVMPartitionTypeHFS (see below) on an unmodified device.
        uint64_t guid[2]; //Random.
        uint64_t begin; //Partitions begin in bytes.
        uint64_t end; //End.
        uint64_t attribute; // 0 == unencrypted; 0x1000000000000 == encrypted
        uint16_t partitionName[0x24]; // UTF-16 encoded
} __attribute__ ((packed)) LwVMPartitionRecord;

typedef struct _LwVM {
        uint64_t type[2]; //Should be LwVMType or LwVMType_noCRC.
        uint64_t guid[2]; //Random.
        uint64_t mediaSize; //Size in bytes.
        uint32_t numPartitions; //Number of partitions.
        uint32_t crc32;
        uint8_t unkn[464]; //Some unknown bytes, usually nulls.
        LwVMPartitionRecord partitions[12];
        uint16_t chunks[1024]; // chunks[0] should be 0xF000
} __attribute__ ((packed)) LwVM;

static const char LwVMType[] = { 0x6A, 0x90, 0x88, 0xCF, 0x8A, 0xFD, 0x63, 0x0A, 0xE3, 0x51, 0xE2, 0x48, 0x87, 0xE0, 0xB9, 0x8B };

static const char LwVMType_noCRC[] = { 0xB1, 0x89, 0xA5, 0x19, 0x4F, 0x59, 0x4B, 0x1D, 0xAD, 0x44, 0x1E, 0x12, 0x7A, 0xAF, 0x45, 0x39 };

static const char LwVMPartitionTypeHFS[] = { 0x48, 0x46, 0x53, 0x00, 0x00, 0x00, 0x11, 0xAA, 0xAA, 0x11, 0x00, 0x30, 0x65, 0x43, 0xEC, 0xAC };

LightweightVolumeManager IOService

Running 'ioreg | grep "Lightweight"' shows, that there's an IOService with such name. I tried fuzzing IOConnectCallStructMethod selectors (very bad idea). The first call which returned KERN_SUCCESS was done with selector 1. Next, I tried this code:

<...>
kern_ret = IOConnectCallStructMethod(lwvm_conn, 1, NULL, 0, &info_sz, &o_sz);
if (kern_ret == KERN_SUCCESS)
{
           printf("OK.\n");
           void *info = malloc(info_sz);
           IOConnectCallStructMethod(lwvm_conn, 1, NULL, 0, info, &info_sz);
           <...>

All disk read operations started resulting with errors. Then my device rebooted and entered a boot loop. The code possibly messed the partition table up.

TODO: reverse that IOService, attempt to modify the partition table, add the information here.

Tools

lwvmedit

External links

iPhone dataprotection wiki