Suspend

When the system is suspended, it enters a low power state where the main application processor is turned off. The main DRAM state is preserved so upon resume, the kernel does not have to be reloaded.

Suspending

//NOTE: the structure is memset(0) before usage. All fields marked "Never written to" are thus 0.
//Also, data coming from KBL Param comes from the "new" KBL Param regenerated after boot
typedef struct SceKernelResumeInfo { //Size is 0x40 in 0.931.010 and 3.60
    SceSize size;               // Size of this structure
    SceUInt32 wakeupFactor;     // from KBL Param
    SceUInt32 unk0x8;           // Never written to
    SceUInt32 sleepFactor;      // from KBL Param
    SceUInt32 unk0x10;          // Never written to
    SceUInt32 bootControlsInfo; // from KBL Param
    SceUInt32 kblParam_0xC8;    // from KBL Param
    SceUInt32 unk0x1C[9];       // Never written to
} SceKernelResumeInfo;

To suspend the system, the Vita saves the current context into a buffer and sends it to Syscon. Then it issues a Syscon request to enter the low-power state.

Context Buffer

Offset	Size	Description
0x0	0x4	Size of this structure (0x68)
0x4	0x4	Mode. Same as in smc_set_power_mode call
0x8	0x4	Virtual address of buffer that receives a copy of SceKernelResumeInfo
0xC	0x4	Virtual address of resume function called with the following arguments: (u32 TTBR1, u32 CONTEXTIDR, u32 DACR)
0x10	0x4	Saved SCTLR
0x14	0x4	Saved ACTLR
0x18	0x4	Saved CPACR
0x1C	0x4	Saved TTBR0
0x20	0x4	Saved TTBR1
0x24	0x4	Saved TTBCR
0x28	0x4	Saved DACR
0x2C	0x4	Saved PRRR
0x30	0x4	Saved NMRR
0x34	0x4	Saved VBAR
0x38	0x4	Saved CONTEXTIDR
0x3C	0x4	Saved TPIDRURW
0x40	0x4	Saved TPIDRURO
0x44	0x4	Saved TPIDRPRW
0x48	0x18	Unknown/reserved
0x60	0x8	System time

Saving Context

The context buffer defined above is built and the physical address of the context buffer is written to Syscon memory at offset 0xC i.e it calls sceSysconWriteForDriver with sceSysconWriteForDriver(0xC, &paddr, 4);. Then it issues the syscon command to suspend the device.

Resuming

Early boot process

When resume is launched, Syscon starts first_loader, which runs second_loader, like a cold boot.

The physical address of the context buffer is read from Syscon memory at offset 0xC i.e second_loader calls sceSysconRead(0xC, &paddr, 4);. This physical address is written to KBL Param.

SKBL

Upon power up, the Boot Sequence is the same until the point where the SKBL would jump into the non-secure kernel bootloader at 0x51000000. Instead, it checks the suspendinfo address is valid, then copies resume code (from SKBL .text) to the scratchpad (physical address 0x1F000000), the Suspend Info to 0x1F001000 and creates a SceKernelResumeInfo at 0x1F001800, then enters Non-secure state at 0x1F000000.

suspendinfo_adr check

SKBL performs a sanity check on suspendinfo_adr before attempting to resume with it. The address must be properly aligned, and belong in a specific range of physical address space (namely Non-Secure DRAM). If the check fails, SKBL enters an infinite loop and the boot process halts. The following table details the specifics for different firmwares.

FW version	Minimal physical address	Maximal physical address	Required alignment	Notes
0.931.010	0x40300000	0x5FFFFFFF	0x10	For ES1 rev0/1 Kermit only
0.931.010	0x40300000	0x7FFFFFFF	0x10	For other Kermit revisions
0.940	0x40400000	0x7FFFFFFF	0x8
3.60	0x40200000	0x5FFFFFFF	0x8

SKBL Resume code

This code starts in Non-secure state, Supervisor mode, with the MMU and all caches disabled.

Entrypoint

The entrypoint is very short, it disables interrupts (cpsid if), clears exclusive access requests (clrex) then jumps to the cleanup function at 0x0000000D (alias of 0x1F00000D).

Cleanup function

This function sets SCTLR to 0, clears the Branch Predictor, TLB and Instruction/Data caches, disables alignement check with SCTLR (clear bit 0x2 - this is redundant as 0 was written to SCTLR earlier), then sets up a MMU context before restoring SCTLR (0x20005805, re-enables MMU) and jumping to the restore function at virtual address 0x80000201 (physical address 0x1F000201). This is important as it means TTBR1 now backs the executed code.

The MMU configuration is the following:

• TTBCR is set to 2
• TTBR0 and TTBR1 are set to 0x1F004042
• DACR is set to 0x55555555
• CONTEXTIDR is set to 0xF7

The L1 Page Table is located at 0x1F004000 and only contains 3 Section (1MiB) mappings in domain 0:

• 0x1F000000 is identity mapped
• 0x80000000 is mapped to physical address 0x1F000000
• 0xE2000000 is identity mapped as Strongly Ordered memory

Restore function

This function restores (almost) all the registers from the Suspend Info, then copies the SceKernelResumeInfo to the buffer in Suspend Info, sets r3-r12 to 0xDEADBEEF and jumps to the resume function pointed to by the Suspend Info.

In 0.931.010, no arguments are provided to the resume function (i.e. typedef void (*resumeFunc)(void);). In 0.940.040, DACR is not provided to the resume function. (i.e. typedef void(*resumeFunc)(u32 TTBR1, u32 CONTEXTIDR);).

Non-restored registers

The following registers from the Suspend Info are not restored:

Register	Reason/Notes
ACTLR	Read-only in Non-secure state (SKBL configures it)
TTBR1	Maps the register restore routine
DACR	In firmware 0.931.010-0.940, this gets restored. Not restored in 3.60.

Rebooting with Patches

By abusing the way resume works, we can reboot the device into a custom firmware by patching the non-secure kernel bootloader. The general framework to do this is: first, patch the kernel bootloader in RAM to accept unsigned SELFs and load a custom kprx after psp2bootconfig.skprx. It could be the case that by the time your exploit runs the DRAM region containing the kernel loader has been wiped/reused. If so, you must use a lower level exploit to dump the loader. It may even be possible to dump the compressed version, in that case you wouldn't even need to perform cleanup. Your custom krpx will be loaded after the bare essential kernel modules are loaded (memory management, threads, file IO, etc) and it can then patch the full module loader (SceSblAuthMgr) to accept unsigned SELFs as well as any other patches.

Cleaning up non-secure Kernel Bootloader

Ideally, if we can get the original ARZL compressed version of the non-secure kernel bootloader that is extracted from kernel_boot_loader.self, we can extract it (see ARZL), then we can load that to memory and jump to it from our resume function. However, by the time the system is in a state we control, usually that data would be corrupted. What we are left with is the non-secure kernel bootloader in the state after bootup is completed. In order to get it to run successfully again in our resume function, we need to clean up the data it uses.

1.50 cleanup

Physical Address	Size	Data	Notes
0x51002024	0x4	0x0	Interrupt registering returns error if not 0
0x5100203C	0x4	0x0	Interrupt registering returns error if not 0
0x5100205C	0x4	0x0	Interrupt registering returns error if not 0
0x51002074	0x4	0x0	Interrupt registering returns error if not 0
0x51002090	0x4	0x0	Interrupt registering returns error if not 0
0x510020D0	0x4	0x0	Interrupt registering returns error if not 0
0x51002110	0x4	0x0	Interrupt registering returns error if not 0
0x510021A4	0x4	0x0	Interrupt registering returns error if not 0
0x510023FC	0x4	0x0	Interrupt registering returns error if not 0
0x51002604	0x4	0x0	Interrupt registering returns error if not 0
0x51002A40	0x4	0x0	Interrupt registering returns error if not 0
0x5102A014	0x4	0x0	Eventually gets some pointer
0x5102A020	0x4	0x1	Read Block Count (start at 1)
0x5102A024	0x4	0x1	Set to 0 after psp2bootconfig.skprx loads for hacky lock handling
0x5102A02C	0x4	0xFFFFFFFF	Memory block id after allocation
0x5102A030	0x4	0x600BF34C	Random number set after each boot to ensure integrity
0x5102A034	0x4	0xFF	Boot State ID
0x5102A050	0x4	0xFFFFFFFF	Memory block id after allocation
0x40300100	0x100	KBL Param	KBL Param copied from virtual address 0x46C0

1.69 cleanup

Physical Address	Size	Data	Notes
0x51002A40	0x4	0x0	Interrupt registering returns error if not 0
0x51002024	0x4	0x0	Interrupt registering returns error if not 0
0x5100203C	0x4	0x0	Interrupt registering returns error if not 0
0x5100205C	0x4	0x0	Interrupt registering returns error if not 0
0x51002074	0x4	0x0	Interrupt registering returns error if not 0
0x51002118	0x4	0x0	Interrupt registering returns error if not 0
0x510020D4	0x4	0x0	Interrupt registering returns error if not 0
0x51002090	0x4	0x0	Interrupt registering returns error if not 0
0x51002614	0x4	0x0	Interrupt registering returns error if not 0
0x5100240C	0x4	0x0	Interrupt registering returns error if not 0
0x510021B4	0x4	0x0	Interrupt registering returns error if not 0
0x5102A02C	0x4	0xFFFFFFFF	Memory block id after allocation
0x5102A050	0x4	0xFFFFFFFF	Memory block id after allocation
0x5102A030	0x4	0x600BF34C	Random number set after each boot to ensure integrity
0x5102A024	0x4	0x1	Set to 0 after psp2bootconfig.skprx loads for hacky lock handling
0x40300100	0x100	KBL Param	KBL Param copied from virtual address 0x46C0