linux-kernel - Re: [PATCH 4/4] module: Add hook for security_kernel_post_read

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Message-ID: <1594216064.23056.208.camel@linux.ibm.com>
Date:   Wed, 08 Jul 2020 09:47:44 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     James Morris <jmorris@...ei.org>, Jessica Yu <jeyu@...nel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Scott Branden <scott.branden@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        KP Singh <kpsingh@...gle.com>, Dave Olsthoorn <dave@...aar.me>,
        Hans de Goede <hdegoede@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Paul Moore <paul@...l-moore.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH 4/4] module: Add hook for
 security_kernel_post_read_file()

On Tue, 2020-07-07 at 20:10 -0700, Kees Cook wrote:
> On Tue, Jul 07, 2020 at 08:47:20PM -0400, Mimi Zohar wrote:
> > On Tue, 2020-07-07 at 01:19 -0700, Kees Cook wrote:
> > > Calls to security_kernel_load_data() should be paired with a call to
> > > security_kernel_post_read_file() with a NULL file argument. Add the
> > > missing call so the module contents are visible to the LSMs interested
> > > in measuring the module content. (This also paves the way for moving
> > > module signature checking out of the module core and into an LSM.)
> > > 
> > > Cc: Jessica Yu <jeyu@...nel.org>
> > > Fixes: c77b8cdf745d ("module: replace the existing LSM hook in init_module")
> > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > ---
> > >  kernel/module.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/module.c b/kernel/module.c
> > > index 0c6573b98c36..af9679f8e5c6 100644
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -2980,7 +2980,12 @@ static int copy_module_from_user(const void __user *umod, unsigned long len,
> > >  		return -EFAULT;
> > >  	}
> > >  
> > > -	return 0;
> > > +	err = security_kernel_post_read_file(NULL, (char *)info->hdr,
> > > +					     info->len, READING_MODULE);
> > 
> > There was a lot of push back on calling security_kernel_read_file()
> > with a NULL file descriptor here.[1]  The result was defining a new
> > security hook - security_kernel_load_data - and enumeration -
> > LOADING_MODULE.  I would prefer calling the same pre and post security
> > hook.
> > 
> > Mimi
> > 
> > [1] http://kernsec.org/pipermail/linux-security-module-archive/2018-May/007110.html
> 
> Ah yes, thanks for the pointer to the discussion.
> 
> I think we have four cases then, for differing LSM hooks:
> 
> - no "file", no contents
> 	e.g. init_module() before copying user buffer
> 	security_kernel_load_data()
> - only a "file" available, no contents
> 	e.g. kernel_read_file() before actually reading anything
> 	security_kernel_read_file()
> - "file" and contents
> 	e.g. kernel_read_file() after reading
> 	security_kernel_post_read_file()
> - no "file" available, just the contents
> 	e.g. firmware platform fallback from EFI space (no "file")
> 	unimplemented!
> 
> If an LSM wants to be able to examine the contents of firmware, modules,
> kexec, etc, it needs either a "file" or the full contents.
> 
> The "file" methods all pass through the kernel_read_file()-family. The
> others happen via blobs coming from userspace or (more recently) the EFI
> universe.
> 
> So, if a NULL file is unreasonable, we need, perhaps,
> security_kernel_post_load_data()
> 
> ?

Agreed.

Mimi