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Add a device-mapper target "dm-default-key" which assigns an encryption
key to bios that don't already have one. This will be used on Android's
userdata partition, so that all data not already encrypted with ext4
encryption is still encrypted with a default key (which will not be
derived from a user credential but will still be protected in some way).
Currently this feature depends on inline encryption support in hardware
via Qualcomm ICE; no software fallback is implemented yet.
An alternate approach considered was to introduce a block device ioctl
which would associate an encryption key with a struct block_device,
which would then be used as the default for bios issued to that
block_device. However, that approach had some issues which made the dm
target seem like the better solution, and perhaps more likely to be
accepted upstream in some form someday (at least, once we have
vendor-independent inline encryption support upstream). Specifically:
1.) The struct block_device for a partition really only represents a
part of some underlying disk along with some state associated with
its current users. Notably, the block_device for a given partition
is not guaranteed to stay around while no one has it opened or
mounted. This means that any extra state like an encryption key we
may try to tie a block_device can be lost if there is a period of
time when no one is using the block device. Granted, this can't
happen while the filesystem on the device is mounted, and it also
can't happen on systems that use tmpfs for their /dev because the
block_device will be pinned by the device node. But either way, a
dm target does not have this problem.
2.) The block_device for a partition doesn't have its own request_queue
or queue_limits. One way in which this could cause problems is that
the disk could support discard requests and have discard_zeroes_data
set to true, which specifies that data read back following a discard
is guaranteed to be zeros. That will not be true for an encrypted
partition, so any filesystem that issues zeroouts (which the block
layer may implement with discard) would potentially be broken. We
can handle this correctly in a dm target since each dm device has
its own request_queue and we can disable discard_zeroes_data, just
as dm-crypt does for example.
3.) Since the block layer remaps bios from partitions to the devices
containing them, we'd still need to have ->bi_crypt_key and
initialize it somewhere, e.g. in generic_make_request_checks()
before it does the partition remapping. We can't simply read
->bi_bdev->bd_default_key from the PFK module.
4.) With a block device ioctl, we'd need to carefully handle the cases
where the ioctl is executed while someone else has the block device
open (fail with EBUSY?) or while the block device's mapping already
has pages cached (sync and invalidate them?). This would not be too
difficult, but with a dm target neither of these is a problem.
Change-Id: Ia3884842004cfb84d315ef38e54ab4f35b48cf5f
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Shivaprasad Hongal <shongal@codeaurora.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.