Re: [PATCH v9 13/25] security: Introduce file_release hook

[Stefan Berger]

On 1/15/24 13:17, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the file_release hook.
>
> IMA calculates at file close the new digest of the file content and writes
> it to security.ima, so that appraisal at next file access succeeds.
>
> An LSM could implement an exclusive access scheme for files, only allowing
> access to files that have no references.
>
> The new hook cannot return an error and cannot cause the operation to be
> reverted.
>
> 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 de4a2915bfd4..c72dc75f2bd3 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -385,6 +385,7 @@ static void __fput(struct file *file)
>   	eventpoll_release(file);
>   	locks_remove_file(file);
>   
> +	security_file_release(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 c3fecc7dcb0b..229f84ce12ae 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -173,6 +173,7 @@ 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_release, 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 97f2212c13b6..2997348afcb7 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -395,6 +395,7 @@ 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_release(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_file_ioctl_compat(struct file *file, unsigned int cmd,
> @@ -1008,6 +1009,9 @@ static inline int security_file_alloc(struct file *file)
>   	return 0;
>   }
>   
> +static inline void security_file_release(struct file *file)
> +{ }
> +
>   static inline void security_file_free(struct file *file)
>   { }
>   
> diff --git a/security/security.c b/security/security.c
> index f3d92bffd02f..7d10724872f8 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2724,6 +2724,17 @@ int security_file_alloc(struct file *file)
>   	return rc;
>   }
>   
> +/**
> + * security_file_release() - Perform actions before releasing the file ref
> + * @file: the file
> + *
> + * Perform actions before releasing the last reference to a file.
> + */
> +void security_file_release(struct file *file)
> +{
> +	call_void_hook(file_release, file);
> +}
> +
>   /**
>    * security_file_free() - Free a file's LSM blob
>    * @file: the file