Re: [PATCH v2 16/25] security: Introduce path_post_mknod 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 path_post_mknod 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>
> ---
>  fs/namei.c                    |  5 +++++
>  include/linux/lsm_hook_defs.h |  3 +++
>  include/linux/security.h      |  9 +++++++++
>  security/security.c           | 19 +++++++++++++++++++
>  4 files changed, 36 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 7dc4626859f0..c8c4ab26b52a 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4061,6 +4061,11 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
>  					  dentry, mode, 0);
>  			break;
>  	}
> +
> +	if (error)
> +		goto out2;
> +
> +	security_path_post_mknod(idmap, &path, dentry, mode_stripped, dev);
>  out2:
>  	done_path_create(&path, dentry);
>  	if (retry_estale(error, lookup_flags)) {
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 797f51da3f7d..b1634b5de98c 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -93,6 +93,9 @@ LSM_HOOK(int, 0, path_mkdir, const struct path *dir, struct dentry *dentry,
>  LSM_HOOK(int, 0, path_rmdir, const struct path *dir, struct dentry *dentry)
>  LSM_HOOK(int, 0, path_mknod, const struct path *dir, struct dentry *dentry,
>  	 umode_t mode, unsigned int dev)
> +LSM_HOOK(void, LSM_RET_VOID, path_post_mknod, struct mnt_idmap *idmap,
> +	 const struct path *dir, struct dentry *dentry, umode_t mode,
> +	 unsigned int dev)
>  LSM_HOOK(int, 0, path_truncate, const struct path *path)
>  LSM_HOOK(int, 0, path_symlink, const struct path *dir, struct dentry *dentry,
>  	 const char *old_name)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 7871009d59ae..f210bd66e939 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -1842,6 +1842,9 @@ int security_path_mkdir(const struct path *dir, struct dentry *dentry, umode_t m
>  int security_path_rmdir(const struct path *dir, struct dentry *dentry);
>  int security_path_mknod(const struct path *dir, struct dentry *dentry, umode_t mode,
>  			unsigned int dev);
> +void security_path_post_mknod(struct mnt_idmap *idmap, const struct path *dir,
> +			      struct dentry *dentry, umode_t mode,
> +			      unsigned int dev);
>  int security_path_truncate(const struct path *path);
>  int security_path_symlink(const struct path *dir, struct dentry *dentry,
>  			  const char *old_name);
> @@ -1876,6 +1879,12 @@ static inline int security_path_mknod(const struct path *dir, struct dentry *den
>  	return 0;
>  }
>  
> +static inline void security_path_post_mknod(struct mnt_idmap *idmap,
> +					    const struct path *dir,
> +					    struct dentry *dentry, umode_t mode,
> +					    unsigned int dev)
> +{ }
> +
>  static inline int security_path_truncate(const struct path *path)
>  {
>  	return 0;
> diff --git a/security/security.c b/security/security.c
> index 3e648aa9292c..56c1c1e66fd1 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1702,6 +1702,25 @@ int security_path_mknod(const struct path *dir, struct dentry *dentry,
>  }
>  EXPORT_SYMBOL(security_path_mknod);
>  
> +/**
> + * security_path_post_mknod() - Update inode security field after file creation
> + * @idmap: idmap of the mount
> + * @dir: parent directory
> + * @dentry: new file
> + * @mode: new file mode
> + * @dev: device number
> + *
> + * Update inode security field after a file has been created.
> + */
> +void security_path_post_mknod(struct mnt_idmap *idmap, const struct path *dir,
> +			      struct dentry *dentry, umode_t mode,
> +			      unsigned int dev)
> +{
> +	if (unlikely(IS_PRIVATE(d_backing_inode(dir->dentry))))
> +		return;
> +	call_void_hook(path_post_mknod, idmap, dir, dentry, mode, dev);
> +}
> +
>  /**
>   * security_path_mkdir() - Check if creating a new directory is allowed
>   * @dir: parent directory