Hi, itís been a long time, hasnít it? Life has just been really busy, finals coming up in the following weeks, so yeah, have been preparing for those nasty buggers.
Meh, anyway, so how do isolated loaders work? Asecure_loaders in specific (metldr)? Well, metldr is a raw binary, not an ELF, and here are the segments of it I have figured out at least:
Name Start End
.local_storage_cleanup 00000400 00000860
.text 00000860 0000CB70
.rodata 0000CB70 0000FCD0
.data 0000FCD0 0003E400
.ram 0003E400 00040000
The entrypoint of metldr is at 0◊400, and in essence it just does the following:
ULONG *pStart = (ULONG*)&start;
The start routine prepares the DMA buffer, and essentially is crt0.c, branches to main, then exits. The main routine prepares the global isolated loader constructor (yes, this is C++ code), then branches to loader_start, which sets up the mailbox for recieving mail, and then loads the actual isolated module, after this, it sends back the mail twice, once normally, second with an interrupt. The actual loader decryption subroutine (load_isolated_loader) sets the prepares the SELF for decryption, verifies the header, then gets the program information headers, then verifies each segment. The code for verifying the header essentially sets up a buffer and then calls verify_header. Then metldr loads its AES decryption key, IV, ECDSA public key and curve type then calls verify_header again. Verify_header sets up the buffer manager, and eventually calls verify_signature after running aes_ctr and aes_decrypt. Verify_signature loads the digest, and performs the SHA1 hash checks. Then we verify the signature by using ECDSA signature algorithms. Verify_self_segment loads the elf segment after several buffers are initialized, then the necessary program structures needed for loader initialization are created then control is passed to the cleanup subroutine. This routine essentially zeroes out every register except $r3 (yes, $SP, $LR, $r0-r2, $r4-r127), and branches to the address in $r3. Ta-da! We have successfully decrypted a binary.
Hope this article was useful.