Re: [PATCH 09/24] kexec_file: Disable at runtime if securelevel has been set

[Dave Young]

On 04/07/17 at 03:45am, Mimi Zohar wrote:
> On Fri, 2017-04-07 at 14:19 +0800, Dave Young wrote:
> > On 04/06/17 at 11:49pm, Mimi Zohar wrote:
> > > On Fri, 2017-04-07 at 11:05 +0800, Dave Young wrote:
> > > > On 04/05/17 at 09:15pm, David Howells wrote:
> > > > > From: Chun-Yi Lee <joeyli.kernel@xxxxxxxxx>
> > > > > 
> > > > > When KEXEC_VERIFY_SIG is not enabled, kernel should not loads image
> > > > > through kexec_file systemcall if securelevel has been set.
> > > > > 
> > > > > This code was showed in Matthew's patch but not in git:
> > > > > https://lkml.org/lkml/2015/3/13/778
> 
> I specifically checked to make sure that either kexec_file() signature
> verification was acceptable and would have commented then, if it had
> not been included.
> 
> > > > > Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
> > > > > Signed-off-by: Chun-Yi Lee <jlee@xxxxxxxx>
> > > > > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> > > > > cc: kexec@xxxxxxxxxxxxxxxxxxx
> > > > > ---
> > > > > 
> > > > >  kernel/kexec_file.c |    6 ++++++
> > > > >  1 file changed, 6 insertions(+)
> > > > > 
> > > > > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > > > > index b118735fea9d..f6937eecd1eb 100644
> > > > > --- a/kernel/kexec_file.c
> > > > > +++ b/kernel/kexec_file.c
> > > > > @@ -268,6 +268,12 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > > > >  	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> > > > >  		return -EPERM;
> > > > >  
> > > > > +	/* Don't permit images to be loaded into trusted kernels if we're not
> > > > > +	 * going to verify the signature on them
> > > > > +	 */
> > > > > +	if (!IS_ENABLED(CONFIG_KEXEC_VERIFY_SIG) && kernel_is_locked_down())
> > > > > +		return -EPERM;
> > > > > +
> > > > >  
> > > 
> > > IMA can be used to verify file signatures too, based on the LSM hooks
> > > in  kernel_read_file_from_fd().  CONFIG_KEXEC_VERIFY_SIG should not be
> > > required.
> > 
> > Mimi, I remember we talked somthing before about the two signature 
> > verification. One can change IMA policy in initramfs userspace,
> > also there are kernel cmdline param to disable IMA, so it can break the
> > lockdown? Suppose kexec boot with ima disabled cmdline param and then
> > kexec reboot again..
> 
> Right, we discussed that the same method of measuring the kexec image
> and initramfs, for extending trusted boot to the OS, could also be
> used for verifying the kexec image and initramfs signatures, for
> extending secure boot to the OS.  The file hash would be calculated
> once for both.
> 
> All of your concerns could be addressed with very minor changes to
> IMA.  (Continued in response to David.)

Thanks! As long as IMA can ensure not breaking the lockdown it should be
fine to add an check for either !CONFIG_KEXEC_VERIFY_SIG or !IMA
enforced.

> 
> > > 
> > > > 	/* Make sure we have a legal set of flags */
> > > > >  	if (flags != (flags & KEXEC_FILE_FLAGS))
> > > > >  		return -EINVAL;
> > > > > 
> > > > > 
> > > > > _______________________________________________
> > > > > kexec mailing list
> > > > > kexec@xxxxxxxxxxxxxxxxxxx
> > > > > http://lists.infradead.org/mailman/listinfo/kexec
> > > > 
> > > > Acked-by: Dave Young <dyoung@xxxxxxxxxx>
> > > > 
> > > > Thanks
> > > > Dave
> > > > --
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> > > > 
> > > 
> > 
>