Re: [PATCH v6 07/14] block/crypto: implement the encryption key management

[Max Reitz <mreitz@redhat.com>]

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On 14.05.20 16:14, Daniel P. Berrangé wrote:
> On Thu, May 14, 2020 at 04:09:59PM +0200, Max Reitz wrote:
>> On 10.05.20 15:40, Maxim Levitsky wrote:
>>> This implements the encryption key management using the generic code in
>>> qcrypto layer and exposes it to the user via qemu-img
>>>
>>> This code adds another 'write_func' because the initialization
>>> write_func works directly on the underlying file, and amend
>>> works on instance of luks device.
>>>
>>> This commit also adds a 'hack/workaround' I and Kevin Wolf (thanks)
>>> made to make the driver both support write sharing (to avoid breaking the users),
>>> and be safe against concurrent  metadata update (the keyslots)
>>>
>>> Eventually the write sharing for luks driver will be deprecated
>>> and removed together with this hack.
>>>
>>> The hack is that we ask (as a format driver) for BLK_PERM_CONSISTENT_READ
>>> and then when we want to update the keys, we unshare that permission.
>>> So if someone else has the image open, even readonly, encryption
>>> key update will fail gracefully.
>>>
>>> Also thanks to Daniel Berrange for the idea of
>>> unsharing read, rather that write permission which allows
>>> to avoid cases when the other user had opened the image read-only.
>>>
>>> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
>>> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>>> ---
>>>  block/crypto.c | 127 +++++++++++++++++++++++++++++++++++++++++++++++--
>>>  block/crypto.h |  34 +++++++++++++
>>>  2 files changed, 158 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/block/crypto.c b/block/crypto.c
>>> index 2e16b62bdc..b14cb0ff06 100644
>>> --- a/block/crypto.c
>>> +++ b/block/crypto.c
>>
>> [...]
>>
>>> +static void
>>> +block_crypto_child_perms(BlockDriverState *bs, BdrvChild *c,
>>> +                         const BdrvChildRole *role,
>>> +                         BlockReopenQueue *reopen_queue,
>>> +                         uint64_t perm, uint64_t shared,
>>> +                         uint64_t *nperm, uint64_t *nshared)
>>> +{
>>> +
>>> +    BlockCrypto *crypto = bs->opaque;
>>> +
>>> +    bdrv_filter_default_perms(bs, c, role, reopen_queue,
>>> +            perm, shared, nperm, nshared);
>>> +    /*
>>> +     * Ask for consistent read permission so that if
>>> +     * someone else tries to open this image with this permission
>>> +     * neither will be able to edit encryption keys, since
>>> +     * we will unshare that permission while trying to
>>> +     * update the encryption keys
>>> +     */
>>> +    if (!(bs->open_flags & BDRV_O_NO_IO)) {
>>> +        *nperm |= BLK_PERM_CONSISTENT_READ;
>>> +    }
>>
>> I’m not sure this is important, because this really means we won’t do
>> I/O.  Its only relevant use in this case is for qemu-img info.  Do we
>> really care if someone edits the key slots while qemu-img info is
>> processing?
> 
> FWIW, OpenStack runs  qemu-img info in a periodic background job, so
> it can be concurrent with anything else they are running.

That might actually be a problem then, because this may cause sporadic
failure when trying to change (amend) keyslots; while qemu-img info
holds the CONSISTENT_READ permission, the amend process can’t unshare
it.  That might lead to hard-to-track-down bugs.

> Having said
> that due to previous QEMU bugs, they unconditonally pass the arg to
> qemu-img to explicitly disable locking

Well, then it doesn’t matter in this case.  But still something to
consider, probably.

Max


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