On Fri, Jan 15, 2021 at 10:18:13AM -0800, Eric Biggers wrote: > [This patchset applies to v5.11-rc3] > > Add an ioctl FS_IOC_READ_VERITY_METADATA which allows 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. > > 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. > > This patchset has been tested by new xfstests which call this new ioctl > via a new subcommand for the 'fsverity' program from fsverity-utils. > > Eric Biggers (6): > fs-verity: factor out fsverity_get_descriptor() > fs-verity: don't pass whole descriptor to fsverity_verify_signature() > fs-verity: add FS_IOC_READ_VERITY_METADATA ioctl > fs-verity: support reading Merkle tree with ioctl > fs-verity: support reading descriptor with ioctl > fs-verity: support reading signature with ioctl > > Documentation/filesystems/fsverity.rst | 76 ++++++++++ > fs/ext4/ioctl.c | 7 + > fs/f2fs/file.c | 11 ++ > fs/verity/Makefile | 1 + > fs/verity/fsverity_private.h | 13 +- > fs/verity/open.c | 133 +++++++++++------ > fs/verity/read_metadata.c | 195 +++++++++++++++++++++++++ > fs/verity/signature.c | 20 +-- > include/linux/fsverity.h | 12 ++ > include/uapi/linux/fsverity.h | 14 ++ > 10 files changed, 417 insertions(+), 65 deletions(-) > create mode 100644 fs/verity/read_metadata.c All applied to fscrypt.git#fsverity for 5.12. - Eric
