Re: [Qemu-devel] [PATCH 12/34] hmp: hmp_cont(): don't rely on QERR_DEVICE_ENCRYPTED

[Luiz Capitulino]

On Fri, 10 Aug 2012 10:42:23 +0200
Markus Armbruster <address@hidden> wrote:

> Luiz Capitulino <address@hidden> writes:
> 
> > On Thu, 02 Aug 2012 13:53:08 +0200
> > Markus Armbruster <address@hidden> wrote:
> >
> >> Luiz Capitulino <address@hidden> writes:
> >> 
> >> > This commit changes hmp_cont() to loop through all block devices
> >> > and proactively set an encryption key for any encrypted device
> >> > without a valid one.
> >> >
> >> > This change is needed because QERR_DEVICE_ENCRYPTED is going to be
> >> > dropped by a future commit.
> >> >
> >> > Signed-off-by: Luiz Capitulino <address@hidden>
> >> > ---
> >> >  hmp.c | 43 +++++++++++++++++++++++++------------------
> >> >  1 file changed, 25 insertions(+), 18 deletions(-)
> >> >
> >> > diff --git a/hmp.c b/hmp.c
> >> > index 6b72a64..1ebeb63 100644
> >> > --- a/hmp.c
> >> > +++ b/hmp.c
> >> > @@ -610,34 +610,41 @@ void hmp_pmemsave(Monitor *mon, const QDict *qdict)
> >> >  
> >> >  static void hmp_cont_cb(void *opaque, int err)
> >> >  {
> >> > -    Monitor *mon = opaque;
> >> > -
> >> >      if (!err) {
> >> > -        hmp_cont(mon, NULL);
> >> > +        qmp_cont(NULL);
> >> >      }
> >> >  }
> >> >  
> >> > -void hmp_cont(Monitor *mon, const QDict *qdict)
> >> > +static bool blockinfo_is_encrypted(const BlockInfo *bdev)
> >> >  {
> >> > -    Error *errp = NULL;
> >> > -
> >> > -    qmp_cont(&errp);
> >> > -    if (error_is_set(&errp)) {
> >> > -        if (error_is_type(errp, QERR_DEVICE_ENCRYPTED)) {
> >> > -            const char *device;
> >> > +    return (bdev->inserted && bdev->inserted->encrypted);
> >> > +}
> >> >  
> >> > -            /* The device is encrypted. Ask the user for the password
> >> > -               and retry */
> >> > +static bool blockinfo_key_is_set(const BlockInfo *bdev)
> >> > +{
> >> > +    return (bdev->inserted && bdev->inserted->valid_encryption_key);
> >> > +}
> >> >  
> >> > -            device = error_get_field(errp, "device");
> >> > -            assert(device != NULL);
> >> > +void hmp_cont(Monitor *mon, const QDict *qdict)
> >> > +{
> >> > +    BlockInfoList *bdev_list, *bdev;
> >> > +    Error *errp = NULL;
> >> >  
> >> > -            monitor_read_block_device_key(mon, device, hmp_cont_cb, 
> >> > mon);
> >> > -            error_free(errp);
> >> > -            return;
> >> > +    bdev_list = qmp_query_block(NULL);
> >> > +    for (bdev = bdev_list; bdev; bdev = bdev->next) {
> >> > +        if (blockinfo_is_encrypted(bdev->value) &&
> >> > +            !blockinfo_key_is_set(bdev->value)) {
> >> > +                monitor_read_block_device_key(mon, bdev->value->device,
> >> > +                                              hmp_cont_cb, NULL);
> >> > +                goto out;
> >> >          }
> >> > -        hmp_handle_error(mon, &errp);
> >> >      }
> >> > +
> >> > +    qmp_cont(&errp);
> >> > +    hmp_handle_error(mon, &errp);
> >> > +
> >> > +out:
> >> > +    qapi_free_BlockInfoList(bdev_list);
> >> >  }
> >> >  
> >> >  void hmp_system_wakeup(Monitor *mon, const QDict *qdict)
> >> 
> >> Quote my previous analysis:
> >> 
> >> Diff makes this change look worse than it is.  Odd: M-x ediff gets it
> >> right.  Anyway, here's how I think it works:
> >> 
> >> Unchanged qmp_cont(): search the bdrv_states for the first encrypted one
> >> without a key.  If found, set err argument to QERR_DEVICE_ENCRYPTED.
> >> Other errors unrelated to encrypted devices are also possible.
> >> 
> >> hmp_cont() before: try qmp_cont().  If we get QERR_DEVICE_ENCRYPTED,
> >> extract the device from the error object, and prompt for its key, with a
> >> callback that retries hmp_cont() if the key was provided.
> >> 
> >> After: search the bdrv_states for an encrypted one without a key.  If
> >> there is none, qmp_cont(), no special error handling.  If there is one,
> >> prompt for its key, with a callback that runs qmp_cont() if the key was
> >> provided.
> >> 
> >> End quote.
> >> 
> >> Two observations:
> >> 
> >> 1. I don't understand how this works for multiple encrypted BDSs without
> >> keys.  If there are any, hmp_cont() starts reading the first one's key,
> >> then returns.  But the callback doesn't start reading the next one's
> >> key.  Please explain.
> >
> > The callback calls qmp_cont(), which will fail. Then the user will enter
> > cont again, and the loop on BlockInfos will run again and the user will
> > be asked for the password of the next image.
> >
> > IOW, each time cont is issued by the user it will ask for the password
> > of a different device.
> >
> > That's the current behavior, and I believe it was also the behavior before
> > I converted cont to the qapi.
> 
> Ugh.  Clunky even for QEMU standards.
> 
> cont gives no indication that the run state change didn't happen.

Agreed. We should have freedom to change cont's semantics on HMP if we
need/want to in order to fix this.

But that's out of the scope of this series, of course.

> 
> >> 2. qmp_cont() uses bdrv_key_required() to test whether a BDS lacks a
> >> key.  Your new hmp_cont() uses blockinfo_is_encrypted() &&
> >> !blockinfo_key_is_set().  Not obvious that the two are equivalent.
> >> 
> >> I'm afraid they are not.  bdrv_key_required() checks the backing image
> >> first:
> >> 
> >>     int bdrv_key_required(BlockDriverState *bs)
> >>     {
> >>         BlockDriverState *backing_hd = bs->backing_hd;
> >> 
> >>         if (backing_hd && backing_hd->encrypted && !backing_hd->valid_key)
> >>             return 1;
> >>         return (bs->encrypted && !bs->valid_key);
> >>     }
> >> 
> >> Your code doesn't:
> >> 
> >>     static bool blockinfo_is_encrypted(const BlockInfo *bdev)
> >>     {
> >>         return (bdev->inserted && bdev->inserted->encrypted);
> >>     }
> >> 
> >>     static bool blockinfo_key_is_set(const BlockInfo *bdev)
> >>     {
> >>         return (bdev->inserted && bdev->inserted->valid_encryption_key);
> >>     }
> >> 
> >>     BlockInfoList *qmp_query_block(Error **errp)
> >>     {
> >>         BlockInfoList *head = NULL, *cur_item = NULL;
> >>         BlockDriverState *bs;
> >> 
> >>         QTAILQ_FOREACH(bs, &bdrv_states, list) {
> >>             BlockInfoList *info = g_malloc0(sizeof(*info));
> >> [...]
> >>             if (bs->drv) {
> >>                 info->value->has_inserted = true;
> >>                 info->value->inserted = 
> >> g_malloc0(sizeof(*info->value->inserted));
> >> [...]
> >>                 info->value->inserted->encrypted = bs->encrypted;
> >>                 info->value->inserted->valid_encryption_key = 
> >> bs->valid_key;
> >> [...]
> >> 
> >> Are you sure this is correct?
> >
> > Is it actually possible for backing_hd to be false and valid_key to be true?
> 
> Yes.  Let's create an encrypted QCOW2 image without backing_file:
> 
>     $ qemu-img create -f qcow2 -o encryption,size=1G foo.qcow2
>     Formatting 'foo.qcow2', fmt=qcow2 size=1073741824 encryption=on 
> cluster_size=65536 lazy_refcounts=off 
>     $ qemu-img info foo.qcow2
>     Disk image 'foo.qcow2' is encrypted.
>     password: 
>     image: foo.qcow2
>     file format: qcow2
>     virtual size: 1.0G (1073741824 bytes)
>     disk size: 136K
>     encrypted: yes
>     cluster_size: 65536
>     $ qemu-system-x86_64 -nodefaults --enable-kvm -S -m 512 -vnc :0 -usb 
> -monitor stdio -drive if=ide,file=foo.qcow2,id=foo
>     QEMU 1.1.50 monitor - type 'help' for more information
>     (qemu) info block
>     foo: removable=0 io-status=ok file=foo.qcow2 ro=0 drv=qcow2 encrypted=1 
> bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
>     (qemu) c
>     foo (foo.qcow2) is encrypted.
>     Password: 
> 
> Now wrap an unencrypted one around it:
> 
>     $ qemu-img create -f qcow2 -o size=1G,backing_file=foo.qcow2 bar.qcow2
>     Formatting 'bar.qcow2', fmt=qcow2 size=1073741824 
> backing_file='foo.qcow2' encryption=off cluster_size=65536 lazy_refcounts=off 
>     $ qemu-img info bar.qcow2
>     image: bar.qcow2
>     file format: qcow2
>     virtual size: 1.0G (1073741824 bytes)
>     disk size: 196K
>     cluster_size: 65536
>     backing file: foo.qcow2
>     $ qemu-system-x86_64 -nodefaults --enable-kvm -S -m 512 -vnc :0 -usb 
> -monitor stdio -drive if=ide,file=bar.qcow2,id=foo
>     QEMU 1.1.50 monitor - type 'help' for more information
>     (qemu) c
>     'foo' (foo.qcow2) is encrypted
>     (qemu) 
> 
> Regression :)

Hmm, right. I think this can also be reproduced by passing -snapshot
when using an encrypted image.

> >> I understand we require HMP code to go via QMP for everything, to keep
> >> HMP honest.  This case shows a drawback: duplicated code, leading to
> >> inconsistencies.
> >
> > Keeping DeviceEncrypted would solve this.
> 
> Another way is to replace valid-encryption-key by the predicate that's
> actually wanted: key-required.

Looks good, this would fix the bug above too, right?