linux-kernel - Re: [RFC PATCH v2] fw_lockdown: new micro LSM module to prevent loading unsigned firmware

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Message-Id: <1510601807.3711.16.camel@linux.vnet.ibm.com>
Date:   Mon, 13 Nov 2017 14:36:47 -0500
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Kyle McMartin <kyle@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>
Subject: Re: [RFC PATCH v2] fw_lockdown: new micro LSM module to prevent
 loading unsigned firmware

On Mon, 2017-11-13 at 20:05 +0100, Luis R. Rodriguez wrote:
> On Mon, Nov 13, 2017 at 06:43:34AM -0500, Mimi Zohar wrote:

> > + * fw_lockdown_read_file - prevent loading of unsigned firmware
> > + * @file: pointer to firmware
> > + * @read_id: caller identifier
> > + *
> > + * Prevent loading of unsigned firmware in lockdown mode.
> > + */
> > +static int fw_lockdown_read_file(struct file *file, enum kernel_read_file_id id)
> > +{
> > +	if (id == READING_FIRMWARE) {
> > +		if (!is_ima_appraise_enabled() &&
> > +		    kernel_is_locked_down("Loading of unsigned firmware"))
> > +			return -EACCES;
> > +	}
> 
> How about just if (id != READING_FIRMWARE) return 0 right away so that
> the real code of focus is not always indented.

Sure

> This could let the code
> grow nicely.
> 
> What I meant above is later we may extend this with:
> 
> if hash_available()
> 	if !valid_hash()
> 		return -EACCES
> else if default_fw_key_available()
> 	if !fw_signed_default_key()
> 		return -EACCES;
> 
> That could be the way we support a default system policy for firmware
> signing, and it would not require any modifications to any firmware
> API callers.
> 
> Notice though that if we later want to extend support for custom requirements
> the semantics behind kernel_read_file() would not suffice to LSMify them, as
> such I'd think we'd need another call which lets the security requirements
> be passed.
> 
> Its unclear if IMA may want to ignore that criteria, as it does the checks in
> userspace.

Huh, I kind of lost you here.  What does "it" refer to in the above
sentence?  IMA is in the kernel.  So, who does what checks in
userspace?

>  If it *can* make use of it, it could do the check-in kernel, of
> course.

> > +	return 0;
> > +}
> > +
> > +static struct security_hook_list fw_lockdown_hooks[] = {
> > +	LSM_HOOK_INIT(kernel_read_file, fw_lockdown_read_file)
> > +};
> > +
> > +static int __init init_fw_lockdown(void)
> > +{
> > +	security_add_hooks(fw_lockdown_hooks, ARRAY_SIZE(fw_lockdown_hooks),
> > +			   "fw_lockdown");
> > +	pr_info("initialized\n");
> > +	return 0;
> > +}
> > +
> > +late_initcall(init_fw_lockdown);
> > +MODULE_LICENSE("GPL");
> > diff --git a/security/security.c b/security/security.c
> > index 4bf0f571b4ef..61a0c95ec687 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -32,7 +32,7 @@
> >  #define MAX_LSM_EVM_XATTR	2
> >  
> >  /* Maximum number of letters for an LSM name string */
> > -#define SECURITY_NAME_MAX	10
> > +#define SECURITY_NAME_MAX	15
> 
> Should this small hunk be a separate atomic patch?

I thought about it, but this is the first and only LSM with a larger
name.

Mimi