RE: [PATCH RFC v2 2/5] X86: Support LSM determination of side-channel vulnerability

[Schaufler, Casey]

> -----Original Message-----
> From: Jann Horn [mailto:jannh@xxxxxxxxxx]
> Sent: Tuesday, August 21, 2018 3:20 AM
> To: Schaufler, Casey <casey.schaufler@xxxxxxxxx>
> Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>; kernel list
> <linux-kernel@xxxxxxxxxxxxxxx>; linux-security-module <linux-security-
> module@xxxxxxxxxxxxxxx>; selinux@xxxxxxxxxxxxx; Hansen, Dave
> <dave.hansen@xxxxxxxxx>; Dock, Deneen T <deneen.t.dock@xxxxxxxxx>;
> kristen@xxxxxxxxxxxxxxx; Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH RFC v2 2/5] X86: Support LSM determination of side-
> channel vulnerability
> 
> On Mon, Aug 20, 2018 at 4:45 PM Schaufler, Casey
> <casey.schaufler@xxxxxxxxx> wrote:
> >
> > > -----Original Message-----
> > > From: Jann Horn [mailto:jannh@xxxxxxxxxx]
> > > Sent: Friday, August 17, 2018 4:55 PM
> > > To: Schaufler, Casey <casey.schaufler@xxxxxxxxx>
> > > Cc: Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>; kernel list
> > > <linux-kernel@xxxxxxxxxxxxxxx>; linux-security-module <linux-security-
> > > module@xxxxxxxxxxxxxxx>; selinux@xxxxxxxxxxxxx; Hansen, Dave
> > > <dave.hansen@xxxxxxxxx>; Dock, Deneen T <deneen.t.dock@xxxxxxxxx>;
> > > kristen@xxxxxxxxxxxxxxx; Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> > > Subject: Re: [PATCH RFC v2 2/5] X86: Support LSM determination of side-
> > > channel vulnerability
> > >
> > > On Sat, Aug 18, 2018 at 12:17 AM Casey Schaufler
> > > <casey.schaufler@xxxxxxxxx> wrote:
> > > >
> > > > From: Casey Schaufler <cschaufler@xxxxxxxxxxxxxxxxxxxxx>
> > > >
> > > > When switching between tasks it may be necessary
> > > > to set an indirect branch prediction barrier if the
> > > > tasks are potentially vulnerable to side-channel
> > > > attacks. This adds a call to security_task_safe_sidechannel
> > > > so that security modules can weigh in on the decision.
> > > >
> > > > Signed-off-by: Casey Schaufler <casey.schaufler@xxxxxxxxx>
> > > > ---
> > > >  arch/x86/mm/tlb.c | 12 ++++++++----
> > > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > > > index 6eb1f34c3c85..8714d4af06aa 100644
> > > > --- a/arch/x86/mm/tlb.c
> > > > +++ b/arch/x86/mm/tlb.c
> > > > @@ -7,6 +7,7 @@
> > > >  #include <linux/export.h>
> > > >  #include <linux/cpu.h>
> > > >  #include <linux/debugfs.h>
> > > > +#include <linux/security.h>
> > > >
> > > >  #include <asm/tlbflush.h>
> > > >  #include <asm/mmu_context.h>
> > > > @@ -270,11 +271,14 @@ void switch_mm_irqs_off(struct mm_struct
> *prev,
> > > struct mm_struct *next,
> > > >                  * threads. It will also not flush if we switch to idle
> > > >                  * thread and back to the same process. It will flush if we
> > > >                  * switch to a different non-dumpable process.
> > > > +                * If a security module thinks that the transition
> > > > +                * is unsafe do the flush.
> > > >                  */
> > > > -               if (tsk && tsk->mm &&
> > > > -                   tsk->mm->context.ctx_id != last_ctx_id &&
> > > > -                   get_dumpable(tsk->mm) != SUID_DUMP_USER)
> > > > -                       indirect_branch_prediction_barrier();
> > > > +               if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id)
> {
> > > > +                       if (get_dumpable(tsk->mm) != SUID_DUMP_USER ||
> > > > +                           security_task_safe_sidechannel(tsk) != 0)
> > > > +                               indirect_branch_prediction_barrier();
> > > > +               }
> > >
> > > When you posted v1 of this series, I asked:
> > >
> > > | Does this enforce transitivity? What happens if we first switch from
> > > | an attacker task to a task without ->mm, and immediately afterwards
> > > | from the task without ->mm to a victim task? In that case, whether a
> > > | flush happens between the attacker task and the victim task depends on
> > > | whether the LSM thinks that the mm-less task should have access to the
> > > | victim task, right?
> > >
> > > Have you addressed that? I don't see it...
> >
> > Nope. That's going to require maintaining state about all the
> > tasks in the chain that might still have cache involvement.
> >
> >         A -> B -> C -> D
> 
> Really?

I am willing to be educated otherwise. My understanding
of Modern Processor Technology will never be so deep that
I won't listen to reason.

> 
> From what I can tell, it'd be enough to:
> 
>  - ensure that the LSM-based access checks behave approximately transitively
>    (which I think they already do, mostly)

Smack rules are explicitly and intentionally not transitive.

A reads B, B reads C does *not* imply A reads C.

>  - keep a copy of the metadata of the last non-kernel task on the CPU

Do you have a suggestion of how one might do that?
I'm willing to believe the information could be available,
but I have yet to come up with a mechanism for getting it.

> 
> > If B and C don't do anything cacheworthy D could conceivably attack A.
> > The amount of state required to detect this case would be prohibitive.
> > I think that if you're sufficiently concerned about this case you should just
> > go ahead and set the barrier. I'm willing to learn something that says I'm
> > wrong.
> 
> That means that an attacker who can e.g. get a CPU to first switch
> from an attacker task to a softirqd (e.g. for network packet
> processing or whatever), then switch from the softirqd to a root-owned
> victim task would be able to bypass the check, right? That doesn't
> sound like a very complicated attack...

Maybe my brain is still stuck in the 1980's, but that sounds pretty
complicated to me! Of course, the fact that it's beyond where I would
go doesn't mean it's implausible.

> 
> I very much dislike the idea of adding a mitigation with a known
> bypass technique to the kernel.

That's fair. I'll look more closely at getting previous_cred_this_cpu().

Thank!