linux-kernel - Re: Kdump with signed images

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Date:	Fri, 02 Nov 2012 14:32:48 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Matthew Garrett <mjg@...hat.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Khalid Aziz <khalid@...ehiking.org>, kexec@...ts.infradead.org,
	horms@...ge.net.au, Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Roberto Sassu <roberto.sassu@...ito.it>,
	Kees Cook <keescook@...omium.org>,
	Peter Jones <pjones@...hat.com>
Subject: Re: Kdump with signed images

Vivek Goyal <vgoyal@...hat.com> writes:

> On Thu, Nov 01, 2012 at 02:52:25PM +0000, Matthew Garrett wrote:
>> On Thu, Nov 01, 2012 at 10:43:04AM -0400, Vivek Goyal wrote:
>> 
>> > So I think this does satisfy the requirement matthew specified. Isn't it?
>> > Matthew, what do you think?
>> 
>> Sure, if you can ensure that. You'll need to figure out how to get the 
>> build system to sign the userspace binaries and you'll need to ensure 
>> that they're statically linked and don't dlopen anything (including the 
>> nsswitch modules), but otherwise that should work.
>> 
>
> [ CC peter jones ]
>
> Ok, so even if we build kexec-tools statically with glibc, we have the
> issue of name service switch modules. glibc will still do dlopen on
> these modules. So what are options now.
>
> - Sign glibc and associated shared libraries. Do not allow unsigned
>   shared library to dynamically link with signed executable.
>
> - Peter mentioned that work with uClibc for kexec-tools.
>
> I personally think that however hard it is but first option sounds like
> a long term solution. We might have more user space processes which
> we might have to trust a generic solution will help with that. For example,
> we might have to sign and trust qemu at some point of time.
>
> Are there other ways of handing glibc issue?

It needs to be checked but /sbin/kexec should not use any functions that
trigger nss switch.  No user or password or host name lookup should be
happening.

This is one part in hardening /sbin/kexec to deal with hostile root
users.  We need to check crazy things like do the files we open on /proc
actually point to /proc after we have opened them.

I believe glibc has some code which triggers for suid root applications
that we should ensure gets triggered that avoid trusting things like
LD_LIBRARY_PATH and company.

Eric
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