Re: [PATCH v17 16/21] fsverity: expose verified fsverity built-in signatures to LSMs

Eric Biggers
2024-04-25T03:36:25Z

On Fri, Apr 12, 2024 at 05:55:59PM -0700, Fan Wu wrote:
> For instance, a policy could be established to permit the execution of all
> files with verified built-in fsverity signatures while restricting kernel
> module loading from specified fsverity files via fsverity digets.

"digets" => "digests"

> The introduction of a security_inode_setintegrity() hook call within
> fsverity's workflow ensures that the verified built-in signature of a file
> is exposed to LSMs, This enables LSMs to recognize and label fsverity files

"LSMs, This" => "LSMs. This"

> +#ifdef CONFIG_FS_VERITY_BUILTIN_SIGNATURES
> +static int fsverity_inode_setintegrity(struct inode *inode,
> + const struct fsverity_descriptor *desc)
> +{
> + return security_inode_setintegrity(inode,
> + LSM_INT_FSVERITY_BUILTINSIG_VALID,
> + desc->signature,
> + le32_to_cpu(desc->sig_size));
> +}
> +#else
> +static inline int fsverity_inode_setintegrity(struct inode *inode,
> + const struct fsverity_descriptor *desc)
> +{
> + return 0;
> +}
> +#endif /* CONFIG_FS_VERITY_BUILTIN_SIGNATURES */
[...]
> @@ -241,6 +259,10 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
> }
> }
>
> + err = fsverity_inode_setintegrity(inode, desc);
> + if (err)
> + goto fail;
> +

Wouldn't it be much simpler to put the LSM call in fsverity_verify_signature()?
Then no #ifdef would be needed, and there would be no weird cases where the LSM
gets passed LSM_INT_FSVERITY_BUILTINSIG_VALID with an empty signature.

> diff --git a/fs/verity/signature.c b/fs/verity/signature.c
> index 90c07573dd77..fd60e9704e78 100644
> --- a/fs/verity/signature.c
> +++ b/fs/verity/signature.c
> @@ -41,7 +41,11 @@ static struct key *fsverity_keyring;
> * @sig_size: size of signature in bytes, or 0 if no signature
> *
> * If the file includes a signature of its fs-verity file digest, verify it
> - * against the certificates in the fs-verity keyring.
> + * against the certificates in the fs-verity keyring. Note that signatures
> + * are verified regardless of the state of the 'fsverity_require_signatures'
> + * variable and the LSM subsystem relies on this behavior to help enforce
> + * file integrity policies. Please discuss changes with the LSM list
> + * (thank you!).
> *
> * Return: 0 on success (signature valid or not required); -errno on failure
> */

... and it would also make the above easier to understand if the LSM call were
to happen right in fsverity_verify_signature().

- Eric