[REPOSTING OLD, CONCEPT] BYPASS Kernel and Ramdisk check for People with UART ACCESS
>>> This was my original post to the other two threads on this concept, I have put this here for completeness. The load address confusion which I had is already resolved <<<
NOTE: THis is based on a initial look at the source code and then the objdump of u-boot.bin. I haven't cross checked this yet, because for now I haven't opened up the nooktab for uart access yet. Also this assumes by default booti command is used for booting in BN uboot. If some one wants to use bootm, then a different location requires to be patched wrt the image loading security check.
If you are a lucky ;) person working with opened up NookTab with UART access, then basically replacing the memory contents of these two offsets with NOP will 90% BYPASS the security check successfully and allow you to boot a MODIFIED KERNEL or RAMDISK as required.
All offsets specified Assuming u-boot is loaded at 0 (adjust for the actual address where u-boot.bin is loaded, haven't looked into that yet).
Check for Security check of Kernel image is at
[ORIG] 0x48c0 => bne 0x48d8 (0x1a00.0004)
Make this a NOP by overwriting using uboot memory write command to
[MODI] 0x48c0 => mov r0, r0 (0xe1a0.0000)
Check for Security check of RAMDisk image is at
[ORIG] 0x4928 => bne 0x4958 (1a00.000a)
Make this a NOP by overwriting with
[MODI] 0x4928 => mov r0, r0 (0xe1a0.0000)
Someone (Hi Adamoutler, maybe you) with opened up NookTab can try this and tell me if it worked or not.
NOTE: you have to add up the actual u-boot load address to the offsets specified.
UPDATE1: It appears the load address is either
Possibility 1) 0x80e8.0000 OR
Possibility 2) 0x80e8.0000-0x120 (More likely).
Have to dig thro bit more, but one of these two will potentially work.
So that means to NOP RAMDisk security check the offset is
Possibility 1 ==> 0x80e8.0000+0x4928
Possibility 2 ==> 0x80e8.0000-0x120+0x4928 (More likely)
Best is to cross check if the resultant address contains the BNE instruction bytes specified above.
Same concept applies for the Kernel security check Nopping offset.
NOTE: It appears there is a 0x120 size header before the actual u-boot.bin code starts and in turn, when I did the objdump, it included the 0x120 bytes of header also assumed as code. And inturn the full (including the header) u-boot.bin or for that matter the u-boot from emmc seems to load into 0x80e8.0000-0x120.
Code around the locations to be noped to help identify the same in memory, in case my offset calculations are wrong
48b4: eb0030f1 bl 0x10c80
48b8: e59d3010 ldr r3, [sp, #16]
48bc: e3530000 cmp r3, #0
48c0: 1a000004 bne 0x48d8
48c4: e59f0104 ldr r0, [pc, #260] ; 0x49d0
48c8: e594100c ldr r1, [r4, #12]
48cc: e5942008 ldr r2, [r4, #8]
48d0: eb0015db bl 0xa044
491c: eb0030d7 bl 0x10c80
4920: e59d3010 ldr r3, [sp, #16]
4924: e3530000 cmp r3, #0
4928: 1a00000a bne 0x4958
492c: e59f00a4 ldr r0, [pc, #164] ; 0x49d8
4930: e5941014 ldr r1, [r4, #20]
4934: e5942010 ldr r2, [r4, #16]
4938: eb0015c1 bl 0xa044
UPDATE 3: ... for a rainy day in future ;)
UPDATE 4: For maximum success, first try a changed RAMDisk rather than Changed Kernel. If Changed Ramdisk works then try Changed Kernel (THere is one more thing in Code, which I am not sure if it will impact a modified kernel or not yet, only way is to experiment).
UPDATE 5: I have cross verified on the target with UART access and the 2nd possibility mentioned above wrt load address is what is correct.