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fs-verity: mention btrfs support

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btrfs supports fs-verity since Linux v5.15.  Document this.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20220610000616.18225-1-ebiggers@kernel.org
This commit is contained in:
Eric Biggers 2022-06-09 17:06:16 -07:00
parent 32346491dd
commit 8da572c52a
2 changed files with 35 additions and 28 deletions
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53
Documentation/filesystems/fsverity.rst
View File

@ -11,9 +11,9 @@ Introduction
fs-verity (``fs/verity/``) is a support layer that filesystems can
hook into to support transparent integrity and authenticity protection
of read-only files. Currently, it is supported by the ext4 and f2fs
filesystems. Like fscrypt, not too much filesystem-specific code is
needed to support fs-verity.
of read-only files. Currently, it is supported by the ext4, f2fs, and
btrfs filesystems. Like fscrypt, not too much filesystem-specific
code is needed to support fs-verity.
fs-verity is similar to `dm-verity
<https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_
@ -473,9 +473,9 @@ files being swapped around.
Filesystem support
==================
fs-verity is currently supported by the ext4 and f2fs filesystems.
The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity
on either filesystem.
fs-verity is supported by several filesystems, described below. The
CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on
any of these filesystems.
``include/linux/fsverity.h`` declares the interface between the
``fs/verity/`` support layer and filesystems. Briefly, filesystems
@ -544,6 +544,13 @@ Currently, f2fs verity only supports a Merkle tree block size of 4096.
Also, f2fs doesn't support enabling verity on files that currently
have atomic or volatile writes pending.
btrfs
-----
btrfs supports fs-verity since Linux v5.15. Verity-enabled inodes are
marked with a RO_COMPAT inode flag, and the verity metadata is stored
in separate btree items.
Implementation details
======================
@ -622,14 +629,14 @@ workqueue, and then the workqueue work does the decryption or
verification. Finally, pages where no decryption or verity error
occurred are marked Uptodate, and the pages are unlocked.
Files on ext4 and f2fs may contain holes. Normally, ``->readahead()``
simply zeroes holes and sets the corresponding pages Uptodate; no bios
are issued. To prevent this case from bypassing fs-verity, these
filesystems use fsverity_verify_page() to verify hole pages.
On many filesystems, files can contain holes. Normally,
``->readahead()`` simply zeroes holes and sets the corresponding pages
Uptodate; no bios are issued. To prevent this case from bypassing
fs-verity, these filesystems use fsverity_verify_page() to verify hole
pages.
ext4 and f2fs disable direct I/O on verity files, since otherwise
direct I/O would bypass fs-verity. (They also do the same for
encrypted files.)
Filesystems also disable direct I/O on verity files, since otherwise
direct I/O would bypass fs-verity.
Userspace utility
=================
@ -648,7 +655,7 @@ Tests
To test fs-verity, use xfstests. For example, using `kvm-xfstests
<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
kvm-xfstests -c ext4,f2fs -g verity
kvm-xfstests -c ext4,f2fs,btrfs -g verity
FAQ
===
@ -771,15 +778,15 @@ weren't already directly answered in other parts of this document.
e.g. magically trigger construction of a Merkle tree.
:Q: Does fs-verity support remote filesystems?
:A: Only ext4 and f2fs support is implemented currently, but in
principle any filesystem that can store per-file verity metadata
can support fs-verity, regardless of whether it's local or remote.
Some filesystems may have fewer options of where to store the
verity metadata; one possibility is to store it past the end of
the file and "hide" it from userspace by manipulating i_size. The
data verification functions provided by ``fs/verity/`` also assume
that the filesystem uses the Linux pagecache, but both local and
remote filesystems normally do so.
:A: So far all filesystems that have implemented fs-verity support are
local filesystems, but in principle any filesystem that can store
per-file verity metadata can support fs-verity, regardless of
whether it's local or remote. Some filesystems may have fewer
options of where to store the verity metadata; one possibility is
to store it past the end of the file and "hide" it from userspace
by manipulating i_size. The data verification functions provided
by ``fs/verity/`` also assume that the filesystem uses the Linux
pagecache, but both local and remote filesystems normally do so.
:Q: Why is anything filesystem-specific at all? Shouldn't fs-verity
be implemented entirely at the VFS level?

10
fs/verity/Kconfig
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@ -14,11 +14,11 @@ config FS_VERITY
help
This option enables fs-verity. fs-verity is the dm-verity
mechanism implemented at the file level. On supported
filesystems (currently EXT4 and F2FS), userspace can use an
ioctl to enable verity for a file, which causes the filesystem
to build a Merkle tree for the file. The filesystem will then
transparently verify any data read from the file against the
Merkle tree. The file is also made read-only.
filesystems (currently ext4, f2fs, and btrfs), userspace can
use an ioctl to enable verity for a file, which causes the
filesystem to build a Merkle tree for the file. The filesystem
will then transparently verify any data read from the file
against the Merkle tree. The file is also made read-only.
This serves as an integrity check, but the availability of the
Merkle tree root hash also allows efficiently supporting