linux-kernel - Re: [PATCH v5 11/23] security: Introduce inode_post

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Message-ID: <85c5dda2-5a2f-4c73-82ae-8a333b69b4a7@schaufler-ca.com>
Date:   Tue, 7 Nov 2023 09:33:10 -0800
From:   Casey Schaufler <casey@...aufler-ca.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,
        paul@...l-moore.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,
        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>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v5 11/23] security: Introduce inode_post_removexattr hook

On 11/7/2023 5:40 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@...wei.com>
>
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the inode_post_removexattr hook.
>
> At inode_removexattr hook, EVM verifies the file's existing HMAC value. At
> inode_post_removexattr, EVM re-calculates the file's HMAC with the passed
> xattr removed and other file metadata.
>
> Other LSMs could similarly take some action after successful xattr removal.
>
> 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>
> ---
>  fs/xattr.c                    |  9 +++++----
>  include/linux/lsm_hook_defs.h |  2 ++
>  include/linux/security.h      |  5 +++++
>  security/security.c           | 14 ++++++++++++++
>  4 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 09d927603433..84a4aa566c02 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -552,11 +552,12 @@ __vfs_removexattr_locked(struct mnt_idmap *idmap,
>  		goto out;
>  
>  	error = __vfs_removexattr(idmap, dentry, name);
> +	if (error)
> +		goto out;

Shouldn't this be simply "return error" rather than a goto to nothing
but "return error"?

>  
> -	if (!error) {
> -		fsnotify_xattr(dentry);
> -		evm_inode_post_removexattr(dentry, name);
> -	}
> +	fsnotify_xattr(dentry);
> +	security_inode_post_removexattr(dentry, name);
> +	evm_inode_post_removexattr(dentry, name);
>  
>  out:
>  	return error;
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 67410e085205..88452e45025c 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -149,6 +149,8 @@ LSM_HOOK(int, 0, inode_getxattr, struct dentry *dentry, const char *name)
>  LSM_HOOK(int, 0, inode_listxattr, struct dentry *dentry)
>  LSM_HOOK(int, 0, inode_removexattr, struct mnt_idmap *idmap,
>  	 struct dentry *dentry, const char *name)
> +LSM_HOOK(void, LSM_RET_VOID, inode_post_removexattr, struct dentry *dentry,
> +	 const char *name)
>  LSM_HOOK(int, 0, inode_set_acl, struct mnt_idmap *idmap,
>  	 struct dentry *dentry, const char *acl_name, struct posix_acl *kacl)
>  LSM_HOOK(int, 0, inode_get_acl, struct mnt_idmap *idmap,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 664df46b22a9..922ea7709bae 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -380,6 +380,7 @@ int security_inode_getxattr(struct dentry *dentry, const char *name);
>  int security_inode_listxattr(struct dentry *dentry);
>  int security_inode_removexattr(struct mnt_idmap *idmap,
>  			       struct dentry *dentry, const char *name);
> +void security_inode_post_removexattr(struct dentry *dentry, const char *name);
>  int security_inode_need_killpriv(struct dentry *dentry);
>  int security_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry);
>  int security_inode_getsecurity(struct mnt_idmap *idmap,
> @@ -940,6 +941,10 @@ static inline int security_inode_removexattr(struct mnt_idmap *idmap,
>  	return cap_inode_removexattr(idmap, dentry, name);
>  }
>  
> +static inline void security_inode_post_removexattr(struct dentry *dentry,
> +						   const char *name)
> +{ }
> +
>  static inline int security_inode_need_killpriv(struct dentry *dentry)
>  {
>  	return cap_inode_need_killpriv(dentry);
> diff --git a/security/security.c b/security/security.c
> index ce3bc7642e18..8aa6e9f316dd 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2452,6 +2452,20 @@ int security_inode_removexattr(struct mnt_idmap *idmap,
>  	return evm_inode_removexattr(idmap, dentry, name);
>  }
>  
> +/**
> + * security_inode_post_removexattr() - Update the inode after a removexattr op
> + * @dentry: file
> + * @name: xattr name
> + *
> + * Update the inode after a successful removexattr operation.
> + */
> +void security_inode_post_removexattr(struct dentry *dentry, const char *name)
> +{
> +	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> +		return;
> +	call_void_hook(inode_post_removexattr, dentry, name);
> +}
> +
>  /**
>   * security_inode_need_killpriv() - Check if security_inode_killpriv() required
>   * @dentry: associated dentry