linux-kernel - Re: [PATCH v4 09/17] LSM: Introduce kernel_post_load

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Date:   Thu, 06 Aug 2020 17:59:46 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Scott Branden <scott.branden@...adcom.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Takashi Iwai <tiwai@...e.de>, Jessica Yu <jeyu@...nel.org>,
        SeongJae Park <sjpark@...zon.de>,
        KP Singh <kpsingh@...omium.org>, linux-efi@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 09/17] LSM: Introduce kernel_post_load_data() hook

On Wed, 2020-07-29 at 10:58 -0700, Kees Cook wrote:
> There are a few places in the kernel where LSMs would like to have
> visibility into the contents of a kernel buffer that has been loaded or
> read. While security_kernel_post_read_file() (which includes the
> buffer) exists as a pairing for security_kernel_read_file(), no such
> hook exists to pair with security_kernel_load_data().
> 
> Earlier proposals for just using security_kernel_post_read_file() with a
> NULL file argument were rejected (i.e. "file" should always be valid for
> the security_..._file hooks, but it appears at least one case was
> left in the kernel during earlier refactoring. (This will be fixed in
> a subsequent patch.)
> 
> Since not all cases of security_kernel_load_data() can have a single
> contiguous buffer made available to the LSM hook (e.g. kexec image
> segments are separately loaded), there needs to be a way for the LSM to
> reason about its expectations of the hook coverage. In order to handle
> this, add a "contents" argument to the "kernel_load_data" hook that
> indicates if the newly added "kernel_post_load_data" hook will be called
> with the full contents once loaded. That way, LSMs requiring full contents
> can choose to unilaterally reject "kernel_load_data" with contents=false
> (which is effectively the existing hook coverage), but when contents=true
> they can allow it and later evaluate the "kernel_post_load_data" hook
> once the buffer is loaded.
> 
> With this change, LSMs can gain coverage over non-file-backed data loads
> (e.g. init_module(2) and firmware userspace helper), which will happen
> in subsequent patches.
> 
> Additionally prepare IMA to start processing these cases.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Thanks, Kees.   Other than a missing "name" field, it looks good.

The security_kernel_post_load_data hook may be used to verify appended
signatures and to measure the buffer data.  Passing the kernel module
(load_info.name) and firmware (fw_name) names is critical at least for
IMA-measurement.

thanks,

Mimi