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

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Date:	Thu, 18 Jun 2015 09:41:35 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Dave Young <dyoung@...hat.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	"Theodore Ts'o" <tytso@....edu>, Petr Tesarik <ptesarik@...e.cz>,
	kexec <kexec@...ts.infradead.org>,
	"Linux-Kernel\@Vger. Kernel. Org" <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: kexec_load(2) bypasses signature verification

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

> 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.

Agreed.

> 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.
>
> 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.

Agreed.

I think disabling kexec_load at compile time can be easily justified.

Anything at runtime is additional complexity, additional bugs,
additional documentation and additional maintenance and needs
to justify itself.

I currently do not see the case for a magic one time runtime disable of
the kexec_load system call.  Maybe there is some valid distro case for
wanting one kernel to do everything and serve every possible need, but I
have not seen that case presented yet.

Eric
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