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>

1 files changed, 11 insertions, 0 deletions

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index f585545277d7..d0aefb5b97fa 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -3357,6 +3357,14 @@ static int f2fs_ioc_measure_verity(struct file *filp, unsigned long arg)
 	return fsverity_ioctl_measure(filp, (void __user *)arg);
 }
 
+static int f2fs_ioc_read_verity_metadata(struct file *filp, unsigned long arg)
+{
+	if (!f2fs_sb_has_verity(F2FS_I_SB(file_inode(filp))))
+		return -EOPNOTSUPP;
+
+	return fsverity_ioctl_read_metadata(filp, (const void __user *)arg);
+}
+
 static int f2fs_ioc_getfslabel(struct file *filp, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -4272,6 +4280,8 @@ static long __f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return f2fs_ioc_enable_verity(filp, arg);
 	case FS_IOC_MEASURE_VERITY:
 		return f2fs_ioc_measure_verity(filp, arg);
+	case FS_IOC_READ_VERITY_METADATA:
+		return f2fs_ioc_read_verity_metadata(filp, arg);
 	case FS_IOC_GETFSLABEL:
 		return f2fs_ioc_getfslabel(filp, arg);
 	case FS_IOC_SETFSLABEL:
@@ -4523,6 +4533,7 @@ long f2fs_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case F2FS_IOC_RESIZE_FS:
 	case FS_IOC_ENABLE_VERITY:
 	case FS_IOC_MEASURE_VERITY:
+	case FS_IOC_READ_VERITY_METADATA:
 	case FS_IOC_GETFSLABEL:
 	case FS_IOC_SETFSLABEL:
 	case F2FS_IOC_GET_COMPRESS_BLOCKS: