Re: [PATCH v2 3/9] security: define security_kernel_read_blob() wrapper

[Casey Schaufler]

On 5/17/2018 7:48 AM, Mimi Zohar wrote:
> In order for LSMs and IMA-appraisal to differentiate between the original
> and new syscalls (eg. kexec, kernel modules, firmware), both the original
> and new syscalls must call an LSM hook.
>
> Commit 2e72d51b4ac3 ("security: introduce kernel_module_from_file hook")
> introduced calling security_kernel_module_from_file() in both the original
> and new syscalls.  Commit a1db74209483 ("module: replace
> copy_module_from_fd with kernel version") replaced these LSM calls with
> security_kernel_read_file().
>
> Commit e40ba6d56b41 ("firmware: replace call to fw_read_file_contents()
> with kernel version") and commit b804defe4297  ("kexec: replace call to
> copy_file_from_fd() with kernel version") replaced their own version of
> reading a file from the kernel with the generic
> kernel_read_file_from_path/fd() versions, which call the pre and post
> security_kernel_read_file LSM hooks.
>
> Missing are LSM calls in the original kexec syscall and firmware sysfs
> fallback method.  From a technical perspective there is no justification
> for defining a new LSM hook, as the existing security_kernel_read_file()
> works just fine.  The original syscalls, however, do not read a file, so
> the security hook name is inappropriate.  Instead of defining a new LSM
> hook, this patch defines security_kernel_read_blob() as a wrapper for
> the existing LSM security_kernel_file_read() hook.

What a marvelous opportunity to bikeshed!

I really dislike adding another security_ interface just because
the name isn't quite right. Especially a wrapper, which is just
code and execution overhead. Why not change security_kernel_read_file()
to security_kernel_read_blob() everywhere and be done?

>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>
> Changelog v2:
> - Define a generic wrapper named security_kernel_read_blob() for
> security_kernel_read_file().
>
> Changelog v1:
> - Define and call security_kexec_load(), a wrapper for
> security_kernel_read_file().
> ---
>  include/linux/security.h | 6 ++++++
>  security/security.c      | 6 ++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 63030c85ee19..4db1967a688b 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -323,6 +323,7 @@ int security_kernel_module_request(char *kmod_name);
>  int security_kernel_read_file(struct file *file, enum kernel_read_file_id id);
>  int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
>  				   enum kernel_read_file_id id);
> +int security_kernel_read_blob(enum kernel_read_file_id id);
>  int security_task_fix_setuid(struct cred *new, const struct cred *old,
>  			     int flags);
>  int security_task_setpgid(struct task_struct *p, pid_t pgid);
> @@ -922,6 +923,11 @@ static inline int security_kernel_post_read_file(struct file *file,
>  	return 0;
>  }
>  
> +static inline int security_kernel_read_blob(enum kernel_read_file_id id)
> +{
> +	return 0;
> +}
> +
>  static inline int security_task_fix_setuid(struct cred *new,
>  					   const struct cred *old,
>  					   int flags)
> diff --git a/security/security.c b/security/security.c
> index 68f46d849abe..8f199b2bf4a2 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1044,6 +1044,12 @@ int security_kernel_read_file(struct file *file, enum kernel_read_file_id id)
>  }
>  EXPORT_SYMBOL_GPL(security_kernel_read_file);
>  
> +int security_kernel_read_blob(enum kernel_read_file_id id)
> +{
> +	return security_kernel_read_file(NULL, id);
> +}
> +EXPORT_SYMBOL_GPL(security_kernel_read_blob);
> +
>  int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
>  				   enum kernel_read_file_id id)
>  {