fs-verity: support reading signature with ioctl

Add support for FS_VERITY_METADATA_TYPE_SIGNATURE to
FS_IOC_READ_VERITY_METADATA. This allows a userspace server program to
retrieve the built-in signature (if present) of a verity file for
serving to a client which implements fs-verity compatible verification.
See the patch which introduced FS_IOC_READ_VERITY_METADATA for more
details.

The ability for userspace to read the built-in signatures is also useful
because it allows a system that is using the in-kernel signature
verification to migrate to userspace signature verification.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-7-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: support reading descriptor with ioctl

Add support for FS_VERITY_METADATA_TYPE_DESCRIPTOR to
FS_IOC_READ_VERITY_METADATA. This allows a userspace server program to
retrieve the fs-verity descriptor of a file for serving to a client
which implements fs-verity compatible verification. See the patch which
introduced FS_IOC_READ_VERITY_METADATA for more details.

"fs-verity descriptor" here means only the part that userspace cares
about because it is hashed to produce the file digest. It doesn't
include the signature which ext4 and f2fs append to the
fsverity_descriptor struct when storing it on-disk, since that way of
storing the signature is an implementation detail. The next patch adds
a separate metadata_type value for retrieving the signature separately.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-6-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: support reading Merkle tree with ioctl

Add support for FS_VERITY_METADATA_TYPE_MERKLE_TREE to
FS_IOC_READ_VERITY_METADATA. This allows a userspace server program to
retrieve the Merkle tree of a verity file for serving to a client which
implements fs-verity compatible verification. See the patch which
introduced FS_IOC_READ_VERITY_METADATA for more details.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-5-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: add FS_IOC_READ_VERITY_METADATA ioctl

Add an ioctl FS_IOC_READ_VERITY_METADATA which will allow reading verity
metadata from a file that has fs-verity enabled, including:

- The Merkle tree
- The fsverity_descriptor (not including the signature if present)
- The built-in signature, if present

This ioctl has similar semantics to pread(). It is passed the type of
metadata to read (one of the above three), and a buffer, offset, and
size. It returns the number of bytes read or an error.

Separate patches will add support for each of the above metadata types.
This patch just adds the ioctl itself.

This ioctl doesn't make any assumption about where the metadata is
stored on-disk. It does assume the metadata is in a stable format, but
that's basically already the case:

- The Merkle tree and fsverity_descriptor are defined by how fs-verity
file digests are computed; see the "File digest computation" section
of Documentation/filesystems/fsverity.rst. Technically, the way in
which the levels of the tree are ordered relative to each other wasn't
previously specified, but it's logical to put the root level first.

- The built-in signature is the value passed to FS_IOC_ENABLE_VERITY.

This ioctl is useful because it allows writing a server program that
takes a verity file and serves it to a client program, such that the
client can do its own fs-verity compatible verification of the file.
This only makes sense if the client doesn't trust the server and if the
server needs to provide the storage for the client.

More concretely, there is interest in using this ability in Android to
export APK files (which are protected by fs-verity) to "protected VMs".
This would use Protected KVM (https://lwn.net/Articles/836693), which
provides an isolated execution environment without having to trust the
traditional "host". A "guest" VM can boot from a signed image and
perform specific tasks in a minimum trusted environment using files that
have fs-verity enabled on the host, without trusting the host or
requiring that the guest has its own trusted storage.

Technically, it would be possible to duplicate the metadata and store it
in separate files for serving. However, that would be less efficient
and would require extra care in userspace to maintain file consistency.

In addition to the above, the ability to read the built-in signatures is
useful because it allows a system that is using the in-kernel signature
verification to migrate to userspace signature verification.

Link: https://lore.kernel.org/r/20210115181819.34732-4-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: move structs needed for file signing to UAPI header

Although it isn't used directly by the ioctls,
"struct fsverity_descriptor" is required by userspace programs that need
to compute fs-verity file digests in a standalone way. Therefore
it's also needed to sign files in a standalone way.

Similarly, "struct fsverity_formatted_digest" (previously called
"struct fsverity_signed_digest" which was misleading) is also needed to
sign files if the built-in signature verification is being used.

Therefore, move these structs to the UAPI header.

While doing this, try to make it clear that the signature-related fields
in fsverity_descriptor aren't used in the file digest computation.

Acked-by: Luca Boccassi <luca.boccassi@microsoft.com>
Link: https://lore.kernel.org/r/20201113211918.71883-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: add SHA-512 support

Add SHA-512 support to fs-verity. This is primarily a demonstration of
the trivial changes needed to support a new hash algorithm in fs-verity;
most users will still use SHA-256, due to the smaller space required to
store the hashes. But some users may prefer SHA-512.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs-verity: add UAPI header

Add the UAPI header for fs-verity, including two ioctls:

- FS_IOC_ENABLE_VERITY
- FS_IOC_MEASURE_VERITY

These ioctls are documented in the "User API" section of
Documentation/filesystems/fsverity.rst.

Examples of using these ioctls can be found in fsverity-utils
(https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git).

I've also written xfstests that test these ioctls
(https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfstests-dev.git/log/?h=fsverity).

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>