linux-kernel - Re: kexec_load(2) bypasses signature verification

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Message-ID: <20150619062140.GH4636@dhcp-128-32.nay.redhat.com>
Date:	Fri, 19 Jun 2015 14:21:40 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	"Theodore Ts'o" <tytso@....edu>, Petr Tesarik <ptesarik@...e.cz>,
	kexec <kexec@...ts.infradead.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: kexec_load(2) bypasses signature verification

On 06/18/15 at 09:30am, Vivek Goyal wrote:
> On Thu, Jun 18, 2015 at 10:02:09AM +0800, Dave Young wrote:
> 
> [..]
> > > Or simply add a new config option KEXEC_VERIFY_SIG_FORCE, so we can return
> > > error in kexec_load and print some error message.
> > 
> > Just like below, does this work for you, Ted?
> > 
> > ---
> >  arch/x86/Kconfig |    7 +++++++
> >  kernel/kexec.c   |    9 ++++++++-
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> > 
> > --- linux.orig/arch/x86/Kconfig
> > +++ linux/arch/x86/Kconfig
> > @@ -1755,6 +1755,13 @@ config KEXEC_VERIFY_SIG
> >  	  verification for the corresponding kernel image type being
> >  	  loaded in order for this to work.
> >  
> > +config KEXEC_VERIFY_SIG_FORCE
> > +	bool "Enforce kexec signature verifying"
> > +	depends on KEXEC_VERIFY_SIG
> > +	---help---
> > +	  This option disable kexec_load() syscall, only kexec_file_load
> > + 	  can be used.
> > +
> 
> 
> Hi Dave,
> 
> I think we might not need a new config option. A new config option makes
> it little confusing. KEXEC_VERIFY_SIG already implies KEXEC_VERIFY_SIG_FORCE
> (for new syscall). Now extending it to also mean that it should disable old
> syscall is confusing.

Hmm, it is only reasonable when kexec_file_load can support bypassing sig
verifying even when CONFIG_KEXEC_VERIFY_SIG=y.

So agree it is confusing to add a _FORCE new option now.

> 
> We already have a sysctl knob to disable kexec kernel loading. But that
> knob disables it on both the syscalls.
> 
> May be we can just introduce another command line option say
> "kexec_verify_sig_force" and this will work across both the syscalls and
> will deny loading a unsigned kernel in following two cases.
> 
> - Using old syscall
> - Using new syscall if kernel was compiled with KEXEC_VERIFY_SIG=n.

As you said KEXEC_VERIFY_SIG implies KEXEC_VERIFY_SIG_FORCE now so if one
disable it in .config, we have no reason to disable kernel loading without
signature verifying? 

> 
> This should be simple and get us going in short term.
> 
> If we want to disable unsigned kernel loading at compile time, then we
> really need to work on decoupling CONFIG_KEXEC and CONFIG_FILE_KEXEC.
> Introducing another config option is not the way forward, IMHO.

Yes, let's do it in this way since everyone is fine with it.

Thanks
Dave
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