Re: [Qemu-devel] [PATCH 1/3] virtio: Basic implementation of virtio pstore driver

[Kees Cook]

On Sun, Jul 17, 2016 at 10:50 PM, Namhyung Kim <address@hidden> wrote:
> Hello,
>
> On Sun, Jul 17, 2016 at 10:12:26PM -0700, Kees Cook wrote:
>> On Sun, Jul 17, 2016 at 9:37 PM, Namhyung Kim <address@hidden> wrote:
>> > The virtio pstore driver provides interface to the pstore subsystem so
>> > that the guest kernel's log/dump message can be saved on the host
>> > machine.  Users can access the log file directly on the host, or on the
>> > guest at the next boot using pstore filesystem.  It currently deals with
>> > kernel log (printk) buffer only, but we can extend it to have other
>> > information (like ftrace dump) later.
>> >
>> > It supports legacy PCI device using single order-2 page buffer.  As all
>> > operation of pstore is synchronous, it would be fine IMHO.  However I
>> > don't know how to make write operation synchronous since it's called
>> > with a spinlock held (from any context including NMI).
>> >
>> > Cc: Paolo Bonzini <address@hidden>
>> > Cc: Radim Kr??m???? <address@hidden>
>> > Cc: "Michael S. Tsirkin" <address@hidden>
>> > Cc: Anthony Liguori <address@hidden>
>> > Cc: Anton Vorontsov <address@hidden>
>> > Cc: Colin Cross <address@hidden>
>> > Cc: Kees Cook <address@hidden>
>> > Cc: Tony Luck <address@hidden>
>> > Cc: Steven Rostedt <address@hidden>
>> > Cc: Ingo Molnar <address@hidden>
>> > Cc: Minchan Kim <address@hidden>
>> > Cc: address@hidden
>> > Cc: address@hidden
>> > Cc: address@hidden
>> > Signed-off-by: Namhyung Kim <address@hidden>
>>
>> This looks great to me! I'd love to use this in qemu. (Right now I go
>> through hoops to use the ramoops backend for testing.)
>>
>> Reviewed-by: Kees Cook <address@hidden>
>
> Thank you!
>
>>
>> Notes below...
>>
>
> [SNIP]
>> > +static u16 to_virtio_type(struct virtio_pstore *vps, enum pstore_type_id 
>> > type)
>> > +{
>> > +       u16 ret;
>> > +
>> > +       switch (type) {
>> > +       case PSTORE_TYPE_DMESG:
>> > +               ret = cpu_to_virtio16(vps->vdev, VIRTIO_PSTORE_TYPE_DMESG);
>> > +               break;
>> > +       default:
>> > +               ret = cpu_to_virtio16(vps->vdev, 
>> > VIRTIO_PSTORE_TYPE_UNKNOWN);
>> > +               break;
>> > +       }
>>
>> I would love to see this support PSTORE_TYPE_CONSOLE too. It should be
>> relatively easy to add: I think it'd just be another virtio command?
>
> Do you want to append the data to the host file as guest does
> printk()?  I think it needs some kind of buffer management, but it's
> not hard to add IMHO.

Well, with most pstore backends, the buffer size is limited, so it
tends to be a circular buffer of some sort. I think whatever you
choose to do is fine (I saw the various mentions of resource limits in
the qemu part of this thread), as long as the last N bytes of console
can be seen on the host side, where N is some portion of the memory
set aside for the log. (I don't mind the idea of an unlimited console
log either, but I suspect that will not be accepted on the qemu
side...)

> [SNIP]
>> > +static int notrace virt_pstore_write(enum pstore_type_id type,
>> > +                                    enum kmsg_dump_reason reason,
>> > +                                    u64 *id, unsigned int part, int count,
>> > +                                    bool compressed, size_t size,
>> > +                                    struct pstore_info *psi)
>> > +{
>> > +       struct virtio_pstore *vps = psi->data;
>> > +       struct virtio_pstore_hdr *hdr = &vps->hdr;
>> > +       struct scatterlist sg[2];
>> > +       unsigned int flags = compressed ? VIRTIO_PSTORE_FL_COMPRESSED : 0;
>> > +
>> > +       *id = vps->id++;
>> > +
>> > +       hdr->cmd   = cpu_to_virtio16(vps->vdev, VIRTIO_PSTORE_CMD_WRITE);
>> > +       hdr->id    = cpu_to_virtio64(vps->vdev, *id);
>> > +       hdr->flags = cpu_to_virtio32(vps->vdev, flags);
>> > +       hdr->type  = to_virtio_type(vps, type);
>> > +
>> > +       sg_init_table(sg, 2);
>> > +       sg_set_buf(&sg[0], hdr, sizeof(*hdr));
>> > +       sg_set_buf(&sg[1], psi->buf, size);
>> > +       virtqueue_add_outbuf(vps->vq, sg, 2, vps, GFP_ATOMIC);
>> > +       virtqueue_kick(vps->vq);
>> > +
>> > +       /* TODO: make it synchronous */
>> > +       return 0;
>>
>> The down side to this being asynchronous is the lack of error
>> reporting. Perhaps this could check hdr->type before queuing and error
>> for any VIRTIO_PSTORE_TYPE_UNKNOWN message instead of trying to send
>> it?
>
> I cannot follow, sorry.  Could you please elaborate it more?

The mention you have here of "TODO: make it synchronous" made me think
about what effects that could have. If a pstore_write() were issued
for a type other than DMESG, the above code would send it through
virtio anyway. No error reporting is possible unless this is
synchronous, but the only error here would simply be "I don't know
what anything except DMESG is", so maybe this code could refuse to
forward anything with type UNKNOWN in the first place. (Just an idea:
I don't think there's anything very wrong here. It just seemed like a
potential improvement.)

>> > +}
>> > +
>> > +static int virt_pstore_erase(enum pstore_type_id type, u64 id, int count,
>> > +                            struct timespec time, struct pstore_info *psi)
>> > +{
>> > +       struct virtio_pstore *vps = psi->data;
>> > +       struct virtio_pstore_hdr *hdr = &vps->hdr;
>> > +       struct scatterlist sg[1];
>> > +       unsigned int len;
>> > +
>> > +       hdr->cmd   = cpu_to_virtio16(vps->vdev, VIRTIO_PSTORE_CMD_ERASE);
>> > +       hdr->id    = cpu_to_virtio64(vps->vdev, id);
>> > +       hdr->type  = to_virtio_type(vps, type);
>> > +
>> > +       sg_init_one(sg, hdr, sizeof(*hdr));
>> > +       virtqueue_add_outbuf(vps->vq, sg, 1, vps, GFP_KERNEL);
>> > +       virtqueue_kick(vps->vq);
>> > +
>> > +       wait_event(vps->acked, virtqueue_get_buf(vps->vq, &len));
>> > +       return 0;
>> > +}
>> > +
>> > +static int virt_pstore_init(struct virtio_pstore *vps)
>> > +{
>> > +       struct pstore_info *psinfo = &vps->pstore;
>> > +       int err;
>> > +
>> > +       vps->id = 0;
>> > +       vps->buflen = 0;
>> > +       psinfo->bufsize = VIRT_PSTORE_BUFSIZE;
>> > +       psinfo->buf = (void *)__get_free_pages(GFP_KERNEL, 
>> > VIRT_PSTORE_ORDER);
>> > +       if (!psinfo->buf) {
>> > +               pr_err("cannot allocate pstore buffer\n");
>> > +               return -ENOMEM;
>> > +       }
>> > +
>> > +       psinfo->owner = THIS_MODULE;
>> > +       psinfo->name  = "virtio";
>> > +       psinfo->open  = virt_pstore_open;
>> > +       psinfo->close = virt_pstore_close;
>> > +       psinfo->read  = virt_pstore_read;
>> > +       psinfo->erase = virt_pstore_erase;
>> > +       psinfo->write = virt_pstore_write;
>> > +       psinfo->flags = PSTORE_FLAGS_FRAGILE;
>>
>> For console support, this flag would need to be dropped -- though I
>> suspect you know that already.:)
>
> Yep, I intentionally support DMESG type only in this patchset for
> simplicity.  Others could be added later. :)

Cool by me. :)

>
>
>>
>> > +       psinfo->data  = vps;
>> > +       spin_lock_init(&psinfo->buf_lock);
>> > +
>> > +       err = pstore_register(psinfo);
>> > +       if (err)
>> > +               kfree(psinfo->buf);
>> > +
>> > +       return err;
>> > +}
>
> [SNIP]
>>
>> Awesome! Can't wait to use it. :)
>
> Thanks for your review! :)

You're welcome! :)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security