Re: [PATCH v2 14/25] security: Introduce file_post_open 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 to move IMA and EVM to the LSM infrastructure, introduce the
> file_post_open hook. Also, export security_file_post_open() for NFS.

Repeat of new LSM hook general comment:
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/namei.c                    |  2 ++
>  fs/nfsd/vfs.c                 |  6 ++++++
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/security.h      |  6 ++++++
>  security/security.c           | 17 +++++++++++++++++
>  5 files changed, 32 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 1f5ec71360de..7dc4626859f0 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3634,6 +3634,8 @@ static int do_open(struct nameidata *nd,
>  	error = may_open(idmap, &nd->path, acc_mode, open_flag);
>  	if (!error && !(file->f_mode & FMODE_OPENED))
>  		error = vfs_open(&nd->path, file);
> +	if (!error)
> +		error = security_file_post_open(file, op->acc_mode);
>  	if (!error)
>  		error = ima_file_check(file, op->acc_mode);
>  	if (!error && do_truncate)
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 8a2321d19194..3450bb1c8a18 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -862,6 +862,12 @@ __nfsd_open(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type,
>  		goto out_nfserr;
>  	}
>  
> +	host_err = security_file_post_open(file, may_flags);
> +	if (host_err) {
> +		fput(file);
> +		goto out_nfserr;
> +	}
> +
>  	host_err = ima_file_check(file, may_flags);
>  	if (host_err) {
>  		fput(file);
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 1153e7163b8b..60ed33f0c80d 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -188,6 +188,7 @@ LSM_HOOK(int, 0, file_send_sigiotask, struct task_struct *tsk,
>  	 struct fown_struct *fown, int sig)
>  LSM_HOOK(int, 0, file_receive, struct file *file)
>  LSM_HOOK(int, 0, file_open, struct file *file)
> +LSM_HOOK(int, 0, file_post_open, struct file *file, int mask)
>  LSM_HOOK(int, 0, file_truncate, struct file *file)
>  LSM_HOOK(int, 0, task_alloc, struct task_struct *task,
>  	 unsigned long clone_flags)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 665bba3e0081..a0f16511c059 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -403,6 +403,7 @@ int security_file_send_sigiotask(struct task_struct *tsk,
>  				 struct fown_struct *fown, int sig);
>  int security_file_receive(struct file *file);
>  int security_file_open(struct file *file);
> +int security_file_post_open(struct file *file, int mask);
>  int security_file_truncate(struct file *file);
>  int security_task_alloc(struct task_struct *task, unsigned long clone_flags);
>  void security_task_free(struct task_struct *task);
> @@ -1044,6 +1045,11 @@ static inline int security_file_open(struct file *file)
>  	return 0;
>  }
>  
> +static inline int security_file_post_open(struct file *file, int mask)
> +{
> +	return 0;
> +}
> +
>  static inline int security_file_truncate(struct file *file)
>  {
>  	return 0;
> diff --git a/security/security.c b/security/security.c
> index 3947159ba5e9..3e0078b51e46 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2856,6 +2856,23 @@ int security_file_open(struct file *file)
>  	return fsnotify_perm(file, MAY_OPEN);
>  }
>  
> +/**
> + * security_file_post_open() - Recheck access to a file after it has been opened
> + * @file: the file
> + * @mask: access mask
> + *
> + * Recheck access with mask after the file has been opened. The hook is useful
> + * for LSMs that require the file content to be available in order to make
> + * decisions.
> + *
> + * Return: Returns 0 if permission is granted.
> + */
> +int security_file_post_open(struct file *file, int mask)
> +{
> +	return call_int_hook(file_post_open, 0, file, mask);
> +}
> +EXPORT_SYMBOL_GPL(security_file_post_open);
> +
>  /**
>   * security_file_truncate() - Check if truncating a file is allowed
>   * @file: file