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Re: [Qemu-devel] [PATCH 02/10] pflash: Macro PFLASH_BUG() is used just o

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH 02/10] pflash: Macro PFLASH_BUG() is used just once, expand
Date: Fri, 22 Feb 2019 08:17:47 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Philippe Mathieu-Daudé <address@hidden> writes:

> On 2/21/19 10:38 AM, Peter Maydell wrote:
>> On Thu, 21 Feb 2019 at 09:22, Markus Armbruster <address@hidden> wrote:
>>> Double-checking... you want me to keep goto reset_flash, like this:
>>> @@ -623,8 +617,8 @@ static void pflash_write(PFlashCFI01 *pfl, hwaddr 
>>> offset,
>>>                  pfl->wcycle = 0;
>>>                  pfl->status |= 0x80;
>>>              } else {
>>> -                DPRINTF("%s: unknown command for \"write block\"\n", 
>>> __func__);
>>> -                PFLASH_BUG("Write block confirm");
>>> +                qemu_log_mask(LOG_GUEST_ERROR,
>>> +                              "unknown command for \"write block\"\n");
>>>                  goto reset_flash;
>>>              }
>>>              break;
>> Yes. (We seem to handle most kinds of guest errors in programming
>> the flash by reset_flash.)
> Oh I missed the context of the patch here.
> So for the case of the Multi-WRITE command (0xe8):

Since I'm a clueless idiot on pflash, I need to process your argument
real slow, so I can write a commit message that doesn't document my
cluelessness forever.

We have a little state machine, and its state is encoded in
pfl->wcycle. pfl->cmd, pfl->counter.  I'm going to show it as

    (value of pfl->wcycle, value of pfl->cmd, value of pfl->counter)

for brevity.

We start with (0, don't care, don't care).

A guest write sends us a width, an address, and a value.
pflash_mem_write_with_attrs() does permission checking, and
pflash_write() the actual work.  We enter it with @offset, @value and
@width holding the message.

    cmd = value;

    trace_pflash_write(offset, value, width, pfl->wcycle);
    if (!pfl->wcycle) {
        /* Set the device in I/O access mode */
        memory_region_rom_device_set_romd(&pfl->mem, false);

@cmd is @value truncated to 8 bits.

> 1/ On first write cycle we have
>   - address = flash_page_address (we store it in pfl->counter)
>   - data = flash_command (0xe8: enter Multi-WRITE)

    switch (pfl->wcycle) {
    case 0:
        /* read mode */
        switch (cmd) {
        case 0xe8: /* Write to buffer */
            DPRINTF("%s: Write to buffer\n", __func__);
            pfl->status |= 0x80; /* Ready! */
        pfl->cmd = cmd;

Transition from (0, don't care, don't care) to (1, 0xE8, don't care).

I can't see "we store it in pfl->counter".

Note that the address (passed in @offset) is entirely ignored.

> 2/ Second cycle:
>   - address = flash_page_address
>     We should check it matches flash_page_address
>     of cycle 1/, but we don't.
>   - data: N
>     "N is the number of elements (bytes / words / double words),
>     minus one, to be written to the write buffer. Expected count
>     ranges are N = 00h to N = 7Fh (e.g., 1 to 128 bytes) in 8-bit
>     mode, N = 00h to N = 003Fh in 16-bit mode, and N = 00h to
>     N = 1Fh in 32-bit mode. Bus cycles 3 and higher are for writing
>     data into the write buffer. The confirm command (D0h) is
>     expected after exactly N + 1 write cycles; any other command at
>     that point in the sequence will prevent the transfer of the
>     buffer to the array (the write will be aborted)."
>     Instead of starting to write the data in a buffer, we write it
>     directly to the block backend.

    case 1:
        switch (pfl->cmd) {
        case 0xe8:
            /* Mask writeblock size based on device width, or bank width if
             * device width not specified.
            if (pfl->device_width) {
                value = extract32(value, 0, pfl->device_width * 8);
            } else {
                value = extract32(value, 0, pfl->bank_width * 8);
            DPRINTF("%s: block write of %x bytes\n", __func__, value);
            pfl->counter = value;

Transition from (1, 0xE8, don't care) to (2, 0xE8, N), where N is passed
in @value.

Again, the address (passed in @offset) is ignored.

Nothing is written to the block backend, yet.

>     Instead of starting to write from cycle 3+, we write now in 2,
>     and keep cycle count == 2 (pfl->wcycle) until all data is
>     written, where we increment at 3.

    case 2:
        switch (pfl->cmd) {
        case 0xe8: /* Block write */
            if (!pfl->ro) {
                pflash_data_write(pfl, offset, value, width, be);
            } else {
                pfl->status |= 0x10; /* Programming error */

Write to memory, with pflash_data_write(), but don't flush to the
backend, yet.  This is (guest-visibly!) wrong.  It's not quite "instead
of starting to write the data in a buffer, we write it directly to the
block backend."

Note that we happily accept any address and width.  I suspect we should
only accept consecutive addresses and consistent width.

            pfl->status |= 0x80;

            if (!pfl->counter) {
                hwaddr mask = pfl->writeblock_size - 1;
                mask = ~mask;

                DPRINTF("%s: block write finished\n", __func__);
                if (!pfl->ro) {
                    /* Flush the entire write buffer onto backing storage.  */
                    pflash_update(pfl, offset & mask, pfl->writeblock_size);
                } else {
                    pfl->status |= 0x10; /* Programming error */

If this is the multi-write's last write, flush the entire write block to
the backend *boggle*.


Transition from (2, 0xE8, N) to

    (2, 0xE8, N-1)          if N>0
    (3, 0xE8, don't care)   if N==0

> 3/ Cycles 3+
>   - address = word index (relative to the page address)
>   - data = word value
>     We check for the CONFIRM command, and switch the device back
>     to READ mode (setting Status), or reset the device (which is
>     modelled the same way).
>     Very silly: If the guest cancelled and never sent the CONFIRM
>     command, the data is already written/flushed back.

    case 3: /* Confirm mode */
        switch (pfl->cmd) {
        case 0xe8: /* Block write */
            if (cmd == 0xd0) {
                pfl->wcycle = 0;
                pfl->status |= 0x80;

On CONFIRM, transition from (3, 0xE8, don't care) back to the intial
state (0, don't care, don't care).

            } else {
                DPRINTF("%s: unknown command for \"write block\"\n", __func__);
                PFLASH_BUG("Write block confirm");
                goto reset_flash;

On anything else, pea brain implodes, and we exit(1).


The fact that we let a hack job like this anywhere near "secure boot" is
either frightening or amusing, depending on one's level of cynicism
towards virtual secure boot.

> So back to the previous code snippet, regardless the value, this
> is neither a hw_error() nor a GUEST_ERROR. This code is simply not
> correct. Eventually the proper (polite) error message should be:
>     qemu_log_mask(LOG_UNIMP, "MULTI_WRITE: Abort not implemented,"
>                              " the data is already written"
>                              " on storage!\n"
>                              "Nevertheless resetting the flash"
>                              " into READ mode.\n");

Yup, makes sense.

Perhaps we should also LOG_UNIMP on the first write of a multi-write
already, because the guest-visible wrongness starts right there.  You
tell me.

Two things left to do for this series: FIXME comments, and a commit
message.  Before I tackle them, I'll give you a chance to correct
misunderstandings in my reasoning.

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