Re: [Qemu-devel] [PATCH 8/9] nvdimm acpi: emulate dsm method

[Xiao Guangrong]

On 03/02/2016 01:09 AM, Michael S. Tsirkin wrote:

> Can't guest trigger this?
> If yes, don't put such code in production please:
> this will fill up disk on the host.

Okay, the evil guest can read the IO port freely. I will use nvdimm_debug() 
instead.

> >   static void
> >   nvdimm_dsm_write(void *opaque, hwaddr addr, uint64_t val, unsigned size)
> >   {
> > +    NvdimmDsmIn *in;
> > +    GArray *out;
> > +    uint32_t buf_size;
> > +    hwaddr dsm_mem_addr = val;
> > +
> > +    nvdimm_debug("dsm memory address %#lx.\n", dsm_mem_addr);
> > +
> > +    /*
> > +     * The DSM memory is mapped to guest address space so an evil guest
> > +     * can change its content while we are doing DSM emulation. Avoid
> > +     * this by copying DSM memory to QEMU local memory.
> > +     */
> > +    in = g_malloc(TARGET_PAGE_SIZE);
> > +    cpu_physical_memory_read(dsm_mem_addr, in, TARGET_PAGE_SIZE);
> > +
> > +    le32_to_cpus(&in->revision);
> > +    le32_to_cpus(&in->function);
> > +    le32_to_cpus(&in->handle);
> > +
> > +    nvdimm_debug("Revision %#x Handler %#x Function %#x.\n", in->revision,
> > +                 in->handle, in->function);
> > +
> > +    out = g_array_new(false, true /* clear */, 1);
> > +
> > +    /*
> > +     * function 0 is called to inquire what functions are supported by
> > +     * OSPM
> > +     */
> > +    if (in->function == 0) {
> > +        build_append_int_noprefix(out, 0 /* No function Supported */,
> > +                                  sizeof(uint8_t));
> > +    } else {
> > +        /* No function is supported yet. */
> > +        build_append_int_noprefix(out, 1 /* Not Supported */,
> > +                                  sizeof(uint8_t));
> > +    }
> > +
> > +    buf_size = cpu_to_le32(out->len);
> > +    cpu_physical_memory_write(dsm_mem_addr, &buf_size, sizeof(buf_size));
>
> is there a race here?
> can guest read this before data is written?

I think no.

It is the SERIALIZED DSM so there is no race in guest. And the CPU has exited
from guest mode when we fill the buffer in the same CPU-context so the guest
can not read the buffer at this point also memory-barrier is not needed here.

> > +    cpu_physical_memory_write(dsm_mem_addr + sizeof(buf_size), out->data,
> > +                              out->len);
>
> What is this doing?
> Is this actually writing AML bytecode into guest memory?

The layout of result written into the buffer is like this:
struct NvdimmDsmOut {
    /* the size of buffer filled by QEMU. */
    uint32_t len;
    uint8_t data[0];
} QEMU_PACKED;
typedef struct NvdimmDsmOut NvdimmDsmOut;

So the first cpu_physical_memory_write() writes the @len and the second one you
pointed out writes the real payload.