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Date:   Tue, 1 Dec 2020 12:55:02 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Peter Zijlstra <peterz@infradead.org>
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        Tim Chen <tim.c.chen@intel.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH -tip 18/32] kernel/entry: Add support for core-wide
 protection of kernel-mode
Message-ID: <20201201175502.GB201514@google.com>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-19-joel@joelfernandes.org>
 <20201125093700.GP2414@hirez.programming.kicks-ass.net>
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On Wed, Nov 25, 2020 at 10:37:00AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 06:19:48PM -0500, Joel Fernandes (Google) wrote:
> > Core-scheduling prevents hyperthreads in usermode from attacking each
> > other, but it does not do anything about one of the hyperthreads
> > entering the kernel for any reason. This leaves the door open for MDS
> > and L1TF attacks with concurrent execution sequences between
> > hyperthreads.
> > 
> > This patch therefore adds support for protecting all syscall and IRQ
> > kernel mode entries. Care is taken to track the outermost usermode exit
> > and entry using per-cpu counters. In cases where one of the hyperthreads
> > enter the kernel, no additional IPIs are sent. Further, IPIs are avoided
> > when not needed - example: idle and non-cookie HTs do not need to be
> > forced into kernel mode.
> > 
> > More information about attacks:
> > For MDS, it is possible for syscalls, IRQ and softirq handlers to leak
> > data to either host or guest attackers. For L1TF, it is possible to leak
> > to guest attackers. There is no possible mitigation involving flushing
> > of buffers to avoid this since the execution of attacker and victims
> > happen concurrently on 2 or more HTs.
> 
> >  .../admin-guide/kernel-parameters.txt         |  11 +
> >  include/linux/entry-common.h                  |  12 +-
> >  include/linux/sched.h                         |  12 +
> >  kernel/entry/common.c                         |  28 +-
> >  kernel/sched/core.c                           | 241 ++++++++++++++++++
> >  kernel/sched/sched.h                          |   3 +
> >  6 files changed, 304 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index bd1a5b87a5e2..b185c6ed4aba 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4678,6 +4678,17 @@
> >  
> >  	sbni=		[NET] Granch SBNI12 leased line adapter
> >  
> > +	sched_core_protect_kernel=
> > +			[SCHED_CORE] Pause SMT siblings of a core running in
> > +			user mode, if at least one of the siblings of the core
> > +			is running in kernel mode. This is to guarantee that
> > +			kernel data is not leaked to tasks which are not trusted
> > +			by the kernel. A value of 0 disables protection, 1
> > +			enables protection. The default is 1. Note that protection
> > +			depends on the arch defining the _TIF_UNSAFE_RET flag.
> > +			Further, for protecting VMEXIT, arch needs to call
> > +			KVM entry/exit hooks.
> > +
> >  	sched_debug	[KNL] Enables verbose scheduler debug messages.
> >  
> >  	schedstats=	[KNL,X86] Enable or disable scheduled statistics.
> 
> So I don't like the parameter name, it's too long. Also I don't like it
> because its a boolean.

Maybe ht_protect= then?

> You're adding syscall,irq,kvm under a single knob where they're all due
> to different flavours of broken. Different hardware might want/need
> different combinations.

Ok, I can try to make it ht_protect=irq,syscall,kvm etc. And conditionally
enable the protection. Does that work for you?
> 
> Hardware without MDS but with L1TF wouldn't need the syscall hook, but
> you're not givng a choice here. And this is generic code, you can't
> assume stuff like this.

Got it.

thanks,

 - Joel