[PATCH v19 15/20] fsverity: expose verified fsverity built-in signatures to LSMs

[Paul Moore]

On May 24, 2024 Fan Wu <wufan at linux.microsoft.com> wrote:
> 
> This patch enhances fsverity's capabilities to support both integrity and
> authenticity protection by introducing the exposure of built-in
> signatures through a new LSM hook. This functionality allows LSMs,
> e.g. IPE, to enforce policies based on the authenticity and integrity of
> files, specifically focusing on built-in fsverity signatures. It enables
> a policy enforcement layer within LSMs for fsverity, offering granular
> control over the usage of authenticity claims. 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 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
> that contain a verified built-in fsverity signature. This hook is invoked
> subsequent to the fsverity_verify_signature() process, guaranteeing the
> signature's verification against fsverity's keyring. This mechanism is
> crucial for maintaining system security, as it operates in kernel space,
> effectively thwarting attempts by malicious binaries to bypass user space
> stack interactions.
> 
> The second to last commit in this patch set will add a link to the IPE
> documentation in fsverity.rst.
> 
> Signed-off-by: Deven Bowers <deven.desai at linux.microsoft.com>
> Signed-off-by: Fan Wu <wufan at linux.microsoft.com>
> ---
> v1-v6:
>   + Not present
> 
> v7:
>   Introduced
> 
> v8:
>   + Split fs/verity/ changes and security/ changes into separate patches
>   + Change signature of fsverity_create_info to accept non-const inode
>   + Change signature of fsverity_verify_signature to accept non-const inode
>   + Don't cast-away const from inode.
>   + Digest functionality dropped in favor of:
>     ("fs-verity: define a function to return the integrity protected
>       file digest")
>   + Reworded commit description and title to match changes.
>   + Fix a bug wherein no LSM implements the particular fsverity @name
>     (or LSM is disabled), and returns -EOPNOTSUPP, causing errors.
> 
> v9:
>   + No changes
> 
> v10:
>   + Rename the signature blob key
>   + Cleanup redundant code
>   + Make the hook call depends on CONFIG_FS_VERITY_BUILTIN_SIGNATURES
> 
> v11:
>   + No changes
> 
> v12:
>   + Add constification to the hook call
> 
> v13:
>   + No changes
> 
> v14:
>   + Add doc/comment to built-in signature verification
> 
> v15:
>   + Add more docs related to IPE
>   + Switch the hook call to security_inode_setintegrity()
> 
> v16:
>   + Explicitly mention "fsverity builtin signatures" in the commit
>     message
>   + Amend documentation in fsverity.rst
>   + Fix format issue
>   + Change enum name
> 
> v17:
>   + Fix various documentation issues
>   + Use new enum name LSM_INT_FSVERITY_BUILTINSIG_VALID
> 
> v18:
>   + Fix typos
>   + Move the inode_setintegrity hook call into fsverity_verify_signature()
> 
> v19:
>   + Cleanup code w.r.t inode_setintegrity hook refactoring
> ---
>  Documentation/filesystems/fsverity.rst | 23 +++++++++++++++++++++--
>  fs/verity/signature.c                  | 18 +++++++++++++++++-
>  include/linux/security.h               |  1 +
>  3 files changed, 39 insertions(+), 3 deletions(-)

...

> diff --git a/fs/verity/signature.c b/fs/verity/signature.c
> index 90c07573dd77..a4ed91c7049f 100644
> --- a/fs/verity/signature.c
> +++ b/fs/verity/signature.c
> @@ -106,6 +111,17 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
>  		return err;
>  	}
>  
> +	err = security_inode_setintegrity(inode,
> +					  LSM_INT_FSVERITY_BUILTINSIG_VALID,
> +					  signature,
> +					  le32_to_cpu(sig_size));

I like this much better without the explicit inode cast :)

> +	if (err) {
> +		fsverity_err(inode, "Error %d exposing file signature to LSMs",
> +			     err);
> +		return err;
> +	}
> +
>  	return 0;
>  }

--
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