Re: kexec_load(2) bypasses signature verification

Dave Young
2015-06-18T02:02:29Z

On 06/18/15 at 09:16am, Dave Young wrote:
> On 06/16/15 at 09:47pm, Vivek Goyal wrote:
> > On Tue, Jun 16, 2015 at 08:32:37PM -0500, Eric W. Biederman wrote:
> > > Vivek Goyal <vgoyal@redhat.com> writes:
> > >
> > > > On Tue, Jun 16, 2015 at 02:38:31PM -0500, Eric W. Biederman wrote:
> > > >>
> > > >> Adding Vivek as he is the one who implemented kexec_file_load.
> > > >> I was hoping he would respond to this thread, and it looks like he
> > > >> simply has not ever been Cc'd.
> > > >>
> > > >> Theodore Ts'o <tytso@mit.edu> writes:
> > > >>
> > > >> > On Mon, Jun 15, 2015 at 09:37:05AM -0400, Josh Boyer wrote:
> > > >> >> The bits that actually read Secure Boot state out of the UEFI
> > > >> >> variables, and apply protections to the machine to avoid compromise
> > > >> >> under the SB threat model. Things like disabling the old kexec...
> > > >> >
> > > >> > I don't have any real interest in using Secure Boot, but I *am*
> > > >> > interested in using CONFIG_KEXEC_VERIFY_SIG[1]. So perhaps we need to
> > > >> > have something similar to what we have with signed modules in terms of
> > > >> > CONFIG_MODULE_SIG_FORCE and module/sig_enforce, but for
> > > >> > KEXEC_VERIFY_SIG. This would mean creating a separate flag
> > > >> > independent of the one Linus suggested for Secure Boot, but since we
> > > >> > have one for signed modules, we do have precedent for this sort of
> > > >> > thing.
> > > >>
> > > >> My overall request with respect to kexec has been that we implement
> > > >> things that make sense outside of the bizarre threat model of the Linux
> > > >> folks who were talking about secure boot.
> > > >>
> > > >> nI have not navigated the labyrinth of config options but having a way to
> > > >> only boot signed things with kexec seems a completely sensible way to
> > > >> operate in the context of signed images.
> > > >>
> > > >> I don't know how much that will help given that actors with sufficient
> > > >> resources have demonstrated the ability to steal private keys, but
> > > >> assuming binary signing is an effective technique (or why else do it)
> > > >> then having an option to limit kexec to only loading signed images seems
> > > >> sensible.
> > > >
> > > > I went through the mail chain on web and here are my thoughts.
> > > >
> > > > - So yes, upstream does not have the logic which automatically disables
> > > > the old syscall (kexec_load()) on secureboot systems. Distributions
> > > > carry those patches.
> > > >
> > > > - This KEXEC_VERIFY_SIG option only cotrols the behavior for
> > > > kexec_file_load() syscall and is not meant to directly affect any
> > > > behavior of old syscall (kexec_load()). I think I should have named
> > > > it KEXEC_FILE_VERIFY_SIG. Though help text makes it clear.
> > > > "Verify kernel signature during kexec_file_load() syscall".
> > > >
> > > > - I think disabling old system call if KEXEC_VERIFY_SIG() is set
> > > > will break existing setup which use old system call by default, except
> > > > the case of secureboot system. And old syscall path is well tested
> > > > and new syscall might not be in a position to support all the corner
> > > > cases, atleast as of now.
> > > >
> > > > Ted,
> > > >
> > > > So looks like you are looking for a system/option where you just want to
> > > > always make use of kexec_file_load() and disable kexec_load(). This sounds
> > > > like you want a kernel where kexec_load() is compiled out and you want
> > > > only kexec_file_load() in.
> > > >
> > > > Right now one can't do that becase kexec_file_load() depends on
> > > > CONFIG_KEXEC option.
> > > >
> > > > I am wondering that how about making CONFIG_KEXEC_FILE_LOAD independent
> > > > of CONFIG_KEXEC. That way one can set CONFIG_KEXEC_VERIFY_SIG=y, and
> > > > only signed kernel can be kexeced on that system.
> > > >
> > > > This should gel well with long term strategy of deprecating kexec_load()
> > > > at some point of time when kexec_file_load() is ready to completely
> > > > replace it.
> > >
> > > Interesting.
> > >
> > > I suspect that what we want is to have CONFIG_KEXEC for the core
> > > and additional CONFIG_KEXEC_LOAD option that covers that kexec_load call.
> > >
> > > That should make it trivially easy to disable the kexec_load system call
> > > in cases where people care.
> >
> > Or, we could create another option CONFIG_KEXEC_CORE/CONFIG_KEXEC_COMMON
> > which will be automatically selected when either CONFIG_KEXEC or
> > CONIG_KEXEC_FILE are selected.
> >
> > All common code can go under this option and rest can go under respective
> > config options.
> >
> > That way, those who have CONFIG_KEXEC=y in old config files will not be
> > broken. They don't have to learn about new options at all.
>
> 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.
+
config KEXEC_BZIMAGE_VERIFY_SIG
bool "Enable bzImage signature verification support"
depends on KEXEC_VERIFY_SIG
--- linux.orig/kernel/kexec.c
+++ linux/kernel/kexec.c
@@ -45,6 +45,12 @@
#include <crypto/hash.h>
#include <crypto/sha.h>

+#ifdef CONFIG_KEXEC_VERIFY_SIG_FORCE
+static bool kexec_verify_sig_force = true;
+#else
+static bool kexec_verify_sig_force;
+#endif
+
/* Per cpu memory for storing cpu states in case of system crash. */
note_buf_t __percpu *crash_notes;

@@ -1243,7 +1249,8 @@ SYSCALL_DEFINE4(kexec_load, unsigned lon
int result;

/* We only trust the superuser with rebooting the system. */
- if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
+ if (!capable(CAP_SYS_BOOT) || kexec_load_disabled
+ || kexec_verify_sig_force)
return -EPERM;

/*