linux-kernel - Re: [PATCH v8 13/24] security: Introduce file

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Date: Fri, 15 Dec 2023 13:41:18 -0800
From: Casey Schaufler <casey@...aufler-ca.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>, viro@...iv.linux.org.uk,
 brauner@...nel.org, chuck.lever@...cle.com, jlayton@...nel.org,
 neilb@...e.de, kolga@...app.com, Dai.Ngo@...cle.com, tom@...pey.com,
 paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com,
 zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, dhowells@...hat.com,
 jarkko@...nel.org, stephen.smalley.work@...il.com, eparis@...isplace.org,
 shuah@...nel.org, mic@...ikod.net
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-nfs@...r.kernel.org, linux-security-module@...r.kernel.org,
 linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
 selinux@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Roberto Sassu <roberto.sassu@...wei.com>,
 Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v8 13/24] security: Introduce file_release hook

On 12/14/2023 9:08 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@...wei.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.
>
> LSMs could also take some action before the last reference of a file is
> released.

You could make this more convincing with an example. Perhaps:

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@...wei.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);
>  	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 e2b45fee94e2..175ca00a6b1d 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 c360458920b1..4c3585e3dcb4 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_mmap_file(struct file *file, unsigned long prot,
> @@ -1006,6 +1007,9 @@ 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 fe6a160afc35..9aa072ca5a19 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