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

[Christian Brauner]

On Mon, Jan 15, 2024 at 07:17:57PM +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu at huawei.com>
> 
> 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 at huawei.com>
> ---
>  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);

This has always been an extremely dicy hook in here and that's caused us
issues before for stacking filesystems so I'm not enthusiastic about
exposing this to all LSMs. So reluctantly,

Acked-by: Christian Brauner <brauner at kernel.org>