Re: [PATCH v2 13/25] security: Introduce inode_post_removexattr hook

[Casey Schaufler <casey@schaufler-ca.com>]

On 8/31/2023 3:41 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
>
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the inode_post_removexattr hook.

This applies to all new LSM hooks:

Would you please include some explanation of how an LSM would use this hook?
You might start with a description of how it is used in IMA/EVM, and why that
could be generally useful.

>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Reviewed-by: Stefan Berger <stefanb@linux.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 e7bbb7f57557..4a0280295686 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;
>  
> -	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 995d30336cfa..1153e7163b8b 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -148,6 +148,8 @@ 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 820899db5276..665bba3e0081 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -374,6 +374,7 @@ 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,
> @@ -919,6 +920,10 @@ 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 764a6f28b3b9..3947159ba5e9 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2354,6 +2354,20 @@ 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