Re: [PATCH v2 15/25] security: Introduce file_pre_free_security hook

[Casey Schaufler]

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

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@xxxxxxxxxx>
> Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---
>  fs/file_table.c               |  1 +
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/security.h      |  4 ++++
>  security/security.c           | 11 +++++++++++
>  4 files changed, 17 insertions(+)
>
> diff --git a/fs/file_table.c b/fs/file_table.c
> index fc7d677ff5ad..964e24120684 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -375,6 +375,7 @@ static void __fput(struct file *file)
>  	eventpoll_release(file);
>  	locks_remove_file(file);
>  
> +	security_file_pre_free(file);
>  	ima_file_free(file);
>  	if (unlikely(file->f_flags & FASYNC)) {
>  		if (file->f_op->fasync)
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 60ed33f0c80d..797f51da3f7d 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -172,6 +172,7 @@ LSM_HOOK(int, 0, kernfs_init_security, struct kernfs_node *kn_dir,
>  	 struct kernfs_node *kn)
>  LSM_HOOK(int, 0, file_permission, struct file *file, int mask)
>  LSM_HOOK(int, 0, file_alloc_security, struct file *file)
> +LSM_HOOK(void, LSM_RET_VOID, file_pre_free_security, struct file *file)
>  LSM_HOOK(void, LSM_RET_VOID, file_free_security, struct file *file)
>  LSM_HOOK(int, 0, file_ioctl, struct file *file, unsigned int cmd,
>  	 unsigned long arg)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index a0f16511c059..7871009d59ae 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -389,6 +389,7 @@ int security_kernfs_init_security(struct kernfs_node *kn_dir,
>  				  struct kernfs_node *kn);
>  int security_file_permission(struct file *file, int mask);
>  int security_file_alloc(struct file *file);
> +void security_file_pre_free(struct file *file);
>  void security_file_free(struct file *file);
>  int security_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
>  int security_mmap_file(struct file *file, unsigned long prot,
> @@ -985,6 +986,9 @@ static inline int security_file_alloc(struct file *file)
>  	return 0;
>  }
>  
> +static inline void security_file_pre_free(struct file *file)
> +{ }
> +
>  static inline void security_file_free(struct file *file)
>  { }
>  
> diff --git a/security/security.c b/security/security.c
> index 3e0078b51e46..3e648aa9292c 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2626,6 +2626,17 @@ int security_file_alloc(struct file *file)
>  	return rc;
>  }
>  
> +/**
> + * security_file_pre_free() - Perform actions before freeing a file's LSM blob
> + * @file: the file
> + *
> + * Perform actions before the file descriptor is freed.
> + */
> +void security_file_pre_free(struct file *file)
> +{
> +	call_void_hook(file_pre_free_security, file);
> +}
> +
>  /**
>   * security_file_free() - Free a file's LSM blob
>   * @file: the file