Re: [PATCH v6 5/5] fsverity: update the documentation

[Stefan Berger]

On 3/18/22 14:21, Mimi Zohar wrote:
> Update the fsverity documentation related to IMA signature support.
>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
>   Documentation/filesystems/fsverity.rst | 22 +++++++++++++---------
>   1 file changed, 13 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst
> index 1d831e3cbcb3..28a47488848e 100644
> --- a/Documentation/filesystems/fsverity.rst
> +++ b/Documentation/filesystems/fsverity.rst
> @@ -74,8 +74,12 @@ authenticating the files is up to userspace.  However, to meet some
>   users' needs, fs-verity optionally supports a simple signature
>   verification mechanism where users can configure the kernel to require
>   that all fs-verity files be signed by a key loaded into a keyring; see
> -`Built-in signature verification`_.  Support for fs-verity file hashes
> -in IMA (Integrity Measurement Architecture) policies is also planned.
> +`Built-in signature verification`_.
> +
> +IMA supports including fs-verity file digests and signatures in the
> +IMA (Integrity Measurement Architecture) measurement list and

The Integrity Measurement Architecture (IMA) supports including ...

> +verifying fs-verity based file signatures stored as security.ima
> +xattrs, based on policy.
>   
>   User API
>   ========
> @@ -653,13 +657,13 @@ weren't already directly answered in other parts of this document.
>       hashed and what to do with those hashes, such as log them,
>       authenticate them, or add them to a measurement list.
>   
> -    IMA is planned to support the fs-verity hashing mechanism as an
> -    alternative to doing full file hashes, for people who want the
> -    performance and security benefits of the Merkle tree based hash.
> -    But it doesn't make sense to force all uses of fs-verity to be
> -    through IMA.  As a standalone filesystem feature, fs-verity
> -    already meets many users' needs, and it's testable like other
> -    filesystem features e.g. with xfstests.
> +    IMA supports the fs-verity hashing mechanism as an alternative
> +    to doing full file hashes, for people who want the performance

IMA supports the fs-verity hashing mechanism as an alternative to full 
file hashes for those who want the performance and security benefits ...

> +    and security benefits of the Merkle tree based hash.  But it
> +    doesn't make sense to force all uses of fs-verity to be through

However, it doesn't make sense ...

> +    IMA.  As a standalone filesystem feature, fs-verity already meets
> +    many users' needs, and it's testable like other filesystem
> +    features e.g. with xfstests.

Fs-verity already meets many user' needs even as a standalone filesystem 
feature and it is testable like other ...

>   
>   :Q: Isn't fs-verity useless because the attacker can just modify the
>       hashes in the Merkle tree, which is stored on-disk?