linux-kernel - Re: [PATCH v5 10/23] security: Introduce inode_post

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Message-ID: <231ff26ec85f437261753faf03b384e6.paul@paul-moore.com>
Date:   Wed, 15 Nov 2023 23:33:45 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chuck.lever@...cle.com, jlayton@...nel.org, neilb@...e.de,
        kolga@...app.com, Dai.Ngo@...cle.com, tom@...pey.com,
        jmorris@...ei.org, serge@...lyn.com, zohar@...ux.ibm.com,
        dmitry.kasatkin@...il.com, dhowells@...hat.com, jarkko@...nel.org,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        casey@...aufler-ca.com, mic@...ikod.net
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        selinux@...r.kernel.org, Roberto Sassu <roberto.sassu@...wei.com>,
        Stefan Berger <stefanb@...ux.ibm.com>
Subject: Re: [PATCH v5 10/23] security: Introduce inode_post_setattr hook

On Nov  7, 2023 Roberto Sassu <roberto.sassu@...weicloud.com> wrote:
> 
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the inode_post_setattr hook.
> 
> At inode_setattr hook, EVM verifies the file's existing HMAC value. At
> inode_post_setattr, EVM re-calculates the file's HMAC based on the modified
> file attributes and other file metadata.
> 
> Other LSMs could similarly take some action after successful file attribute
> change.
> 
> The new hook cannot return an error and cannot cause the operation to be
> reverted.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> Reviewed-by: Stefan Berger <stefanb@...ux.ibm.com>
> Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>
> Acked-by: Casey Schaufler <casey@...aufler-ca.com>
> ---
>  fs/attr.c                     |  1 +
>  include/linux/lsm_hook_defs.h |  2 ++
>  include/linux/security.h      |  7 +++++++
>  security/security.c           | 16 ++++++++++++++++
>  4 files changed, 26 insertions(+)

...

> diff --git a/security/security.c b/security/security.c
> index 7935d11d58b5..ce3bc7642e18 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2222,6 +2222,22 @@ int security_inode_setattr(struct mnt_idmap *idmap,
>  }
>  EXPORT_SYMBOL_GPL(security_inode_setattr);
>  
> +/**
> + * security_inode_post_setattr() - Update the inode after a setattr operation
> + * @idmap: idmap of the mount
> + * @dentry: file
> + * @ia_valid: file attributes set
> + *
> + * Update inode security field after successful setting file attributes.
> + */
> +void security_inode_post_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> +				 int ia_valid)
> +{
> +	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> +		return;

I may be missing it, but I don't see the S_PRIVATE flag check in the
existing IMA or EVM hooks so I'm curious as to why it is added here?
Please don't misunderstand me, I think it makes sense to return early
on private dentrys/inodes, but why aren't we doing that now?

> +	call_void_hook(inode_post_setattr, idmap, dentry, ia_valid);
> +}
> +
>  /**
>   * security_inode_getattr() - Check if getting file attributes is allowed
>   * @path: file
> -- 
> 2.34.1

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