Re: [PATCH v5 12/23] security: Introduce file_post_open hook

[Casey Schaufler]

On 11/7/2023 5:40 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> 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.
>
> Based on policy, IMA calculates the digest of the file content and
> extends the TPM with the digest, verifies the file's integrity based on
> the digest, and/or includes the file digest in the audit log.
>
> LSMs could similarly take action depending on the file content and the
> access mask requested with open().
>
> The new hook returns a value and can cause the open to be aborted.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>

Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>


> ---
>  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 71c13b2990b4..fb93d3e13df6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3620,6 +3620,8 @@ 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 fbbea7498f02..b0c3f07a8bba 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -877,6 +877,12 @@ __nfsd_open(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type,
>  		goto out;
>  	}
>  
> +	host_err = security_file_post_open(file, may_flags);
> +	if (host_err) {
> +		fput(file);
> +		goto out;
> +	}
> +
>  	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 88452e45025c..4f6861fecacd 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -189,6 +189,7 @@ 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 922ea7709bae..c360458920b1 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -409,6 +409,7 @@ 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);
> @@ -1065,6 +1066,11 @@ 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 8aa6e9f316dd..fe6a160afc35 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2954,6 +2954,23 @@ int security_file_open(struct file *file)
>  	return fsnotify_perm(file, MAY_OPEN);
>  }
>  
> +/**
> + * security_file_post_open() - Evaluate a file after it has been opened
> + * @file: the file
> + * @mask: access mask
> + *
> + * Evaluate an opened file and the access mask requested with open(). 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