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Re: [PATCH v2 05/11] block/crypto: implement the encryption key manageme
From: |
Max Reitz |
Subject: |
Re: [PATCH v2 05/11] block/crypto: implement the encryption key management |
Date: |
Fri, 4 Oct 2019 20:41:29 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 |
On 13.09.19 00:30, Maxim Levitsky wrote:
> This implements the encryption key management
> using the generic code in qcrypto layer
> (currently only for qemu-img amend)
>
> This code adds another 'write_func' because the initialization
> write_func works directly on the underlying file,
> because during the creation, there is no open instance
> of the luks driver, but during regular use, we have it,
> and should use it instead.
>
>
> This commit also adds a 'hack/workaround' I and Kevin Wolf (thanks)
> made to make the driver still support write sharing,
> but be safe against concurrent metadata update (the keys)
> Eventually 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 always
> (technically always unless opened with BDRV_O_NO_IO)
>
> and then when we want to update the keys, we
> unshare that permission. So if someone else
> has the image open, even readonly, this will fail.
>
> Also thanks to Daniel Berrange for the variant of
> that hack that involves asking for read,
> rather that write permission
>
> Signed-off-by: Maxim Levitsky <address@hidden>
> ---
> block/crypto.c | 118 +++++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 115 insertions(+), 3 deletions(-)
>
> diff --git a/block/crypto.c b/block/crypto.c
> index a6a3e1f1d8..f42fa057e6 100644
> --- a/block/crypto.c
> +++ b/block/crypto.c
> @@ -36,6 +36,7 @@ typedef struct BlockCrypto BlockCrypto;
>
> struct BlockCrypto {
> QCryptoBlock *block;
> + bool updating_keys;
> };
>
>
> @@ -70,6 +71,24 @@ static ssize_t block_crypto_read_func(QCryptoBlock *block,
> return ret;
> }
>
> +static ssize_t block_crypto_write_func(QCryptoBlock *block,
> + size_t offset,
> + const uint8_t *buf,
> + size_t buflen,
> + void *opaque,
> + Error **errp)
There’s already a function of this name for creation.
> +{
> + BlockDriverState *bs = opaque;
> + ssize_t ret;
> +
> + ret = bdrv_pwrite(bs->file, offset, buf, buflen);
> + if (ret < 0) {
> + error_setg_errno(errp, -ret, "Could not write encryption header");
> + return ret;
> + }
> + return ret;
> +}
> +
>
> struct BlockCryptoCreateData {
> BlockBackend *blk;
[...]
> +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;
> +
> + /*
> + * 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
> + */
> + if (!(bs->open_flags & BDRV_O_NO_IO)) {
> + perm |= BLK_PERM_CONSISTENT_READ;
> + }
> +
> + /*
> + * This driver doesn't modify LUKS metadata except
> + * when updating the encryption slots.
> + * Thus unlike a proper format driver we don't ask for
> + * shared write permission. However we need it
> + * when we area updating keys, to ensure that only we
> + * had opened the device r/w
> + *
> + * Encryption update will set the crypto->updating_keys
> + * during that period and refresh permissions
> + *
> + */
> +
> + if (crypto->updating_keys) {
> + /*need exclusive write access for header update */
> + perm |= BLK_PERM_WRITE;
> + shared &= ~(BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE);
> + }
> +
> + bdrv_filter_default_perms(bs, c, role, reopen_queue,
> + perm, shared, nperm, nshared);
> +}
This will probably work, but usually drivers do it the other way around:
First call any of the default_perms(), and then adjust *nperm and
*nshared as required.
(perm/shared are what the parents need, *nperm/*nshared is what this
driver needs, so it makes more sense that way; and this way nobody has
to check whether the settings survived the default_perms() call.)
((But the permissions themselves do look correct.))
Max
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