i.MX6 and JTAG security violation

[vsiles]

I'm trying to secure a i.MX6 board, to detect any attempt at JTAG debugging, using the SNVS violation mechanism.

I configured the SNVS_HPSICR to trigger interrupt if a JTAG is plugged in, and it works !

Once I detect this event, I'm trying to clean the board's memory before resetting it. So far, I only managed to shut down the board, because each time I'm trying to reboot it (see code below), the board reboots but the violation is still asserted : I can't manage to clean SNVS_HP_SVSR.

All the following code is running at PL1 SVC mode, in (Trustzone) Secure state.

After reading https://community.nxp.com/message/388572, here is what I tried:

- Setting the interrupt as fatal or not: all other entries have been tested with SNVS in non secure or soft fail mode

- I tried to write 0x3f in SNVS_HP_SVSR to clear the w1c bits, it doesn't have any effect

- I tried to unregister the interrupt by cleaning SNVS_HPSICR before cleaning SNVS_HP_SVSR, it doesn't have any effect

- I set SNVS_HPCOMR[NPSWA_EN] to test unprivileged access: I managed to change the bit but it still can't clear SNVS_HP_SVSR.

Here is my latest attempt:

        /* Acknowledge the interrupt ... */
        irq = cpu_regs->acknowledge;
        cpu_regs->endofinterrupt = irq;

        pr_crit("HPCOMR: 0x%" PRIx32 "\n", reg_read32(snvs_base + SNVS_HPCOMR)); /* 0x2002 */
        pr_crit("HPSR: 0x%" PRIx32 "\n", reg_read32(snvs_base + SNVS_HPSR));     /* 0x80000b00 or 0x80000300 */
        pr_crit("LPSR: 0x%" PRIx32 "\n", reg_read32(snvs_base + SNVS_LPSR));     /* 0x40000000 */
        pr_crit("HPSICR: 0x%" PRIx32 "\n", reg_read32(snvs_base + SNVS_HPSICR)); /* 0x17 */
        pr_crit("PANIC IRQ %" PRIu32 " DETECTED !\n", irq);                      /* interrupt 52 */

        /* Current SNVS security mode */
        uint32_t status = reg_read32(snvs_base + SNVS_HPSR);
        status >>= SNVS_HPSR_SSM_ST_OFFSET;
        status &= SNVS_HPSR_SSM_ST_MASK;
        pr_crit("SNVS mode: 0x%" PRIx32 "\n", status); /* 0xb or 0x3 */

        /* Disable all violation interrupts */
        reg_write32(snvs_base + SNVS_HPSICR, 0);
        pr_crit("New HPSICR: 0x%" PRIx32 "\n", reg_read32(snvs_base + SNVS_HPSICR)); /* 0x0 */

        /* dump & ack the violation IRQ in the SNVS */
        status = reg_read32(snvs_base + SNVS_HPSVSR);
        pr_crit("SNVS violation status: 0x%" PRIx32 "\n", status);      /* 0x2 */
        reg_write32(snvs_base + SNVS_HPSVSR, status & UINT32_C(0x3f));
        isb();
        status = reg_read32(snvs_base + SNVS_HPSVSR);
        pr_crit("Checking SNVS status: 0x%" PRIx32 "\n", status);        /* /!\ 0x2, w1c didn't work */

        /* ... and panic */
        panic(PANICL_PANIC_IRQ_VIOLATION);
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This only way I found to clear SNVS_HP_SVSR is to push the hardware reboot button.

To reset the board, I'm using WDOG1:

    reg_clrset16(WDOG1_BASE_PA, UINT16_C(0xff) << 8, UINT16_C(1) << 2);

    // wdog reset sequence
    reg_write16(WDOG1_BASE_PA, UINT16_C(0x5555));
    reg_write16(WDOG1_BASE_PA, UINT16_C(0xaaaa));

    while (1) {
        wfi();
    }
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Can anyone guide me to a way to reboot the board with the violation correctly acknowledged and cleaned up ?

Best,

V.

[marius_grigoras · Accepted solution]

Hi,

You can inspire from this patch (is basically for iMX7D but should work also for iMX6).

Here we implemented a software work-around resetting the board iMX7D_SabreSD using SNVS module

We program the SNVS alarm for 3 seconds to power on system after we issue the SNVS shutdown system, seems something very similar with your needs.

The u-boot branch used for this patch is imx_v2017.03_4.9.11_1.0.0_ga (from [1]).

Best regards,

Marius

[1] uboot-imx - i.MX U-Boot 

[vsiles]

Dear mariusviorelgrigoras-b32331‌, I tried to adapt your patch on my i.MX6 board, and I get the SNVS_LP Alarm interrupt (LPSR[LPTA] is set to 1), but I don't reboot, I resume my execution. The board doesn't seem to have been powered off.

I'll try to check on my side, but by any chance do you know if setting SNVS_LPCR to 0x60 is enough to poweroff the board or did I miss another important step ?

[vsiles]

It seems that my write to SNVS_LPCR is somehow not done correctly:

        uint32_t reg = reg_read32(snvs_base + SNVS_LPCR);
        reg |= UINT32_C(0x62);
        reg_write32(snvs_base + SNVS_LPCR, reg);

        /* wait for reset to assert... */
        pr_msg("waiting for reset to be triggered \n");
        pr_msg("SNVS_LPCR = 0x%" PRIx32 " (wrote 0x%" PRIx32 ")\n",
               reg_read32(snvs_base + SNVS_LPCR), reg);
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This codes displays a value written of 0x63 but a value read in the register of 0x23, the bit TOP "Turn off System Power" should be set until the end of the power off, and it is not set .

[vsiles]

Thanks, I'll have a look.

From what I understand, you program the SNVS_LP alarm to trigger an interrupt after PowerOff has been achieved.

It didn't work out of the box, but I might be missing some configuration so that the interrupt triggers a reboot.

I'll look more into detail to the u-boot branch you linked, thank you !