RE: [Patch v4 16/18] x86/speculation: Enable STIBP to protect security sensitive tasks

[Schaufler, Casey]

> -----Original Message-----
> From: Tim Chen [mailto:tim.c.chen@xxxxxxxxxxxxxxx]
> Sent: Tuesday, October 30, 2018 2:35 PM
> To: Schaufler, Casey <casey.schaufler@xxxxxxxxx>; Jiri Kosina
> <jikos@xxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>; Ingo Molnar
> <mingo@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Josh Poimboeuf
> <jpoimboe@xxxxxxxxxx>; Andrea Arcangeli <aarcange@xxxxxxxxxx>; David
> Woodhouse <dwmw@xxxxxxxxxxxx>; Andi Kleen <ak@xxxxxxxxxxxxxxx>;
> Hansen, Dave <dave.hansen@xxxxxxxxx>; Mallick, Asit K
> <asit.k.mallick@xxxxxxxxx>; Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>; Jon
> Masters <jcm@xxxxxxxxxx>; Waiman Long <longman9394@xxxxxxxxx>;
> linux-kernel@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx
> Subject: Re: [Patch v4 16/18] x86/speculation: Enable STIBP to protect security
> sensitive tasks
> 
> On 10/30/2018 02:07 PM, Schaufler, Casey wrote:
> >> -----Original Message-----
> >> From: Tim Chen [mailto:tim.c.chen@xxxxxxxxxxxxxxx]
> >> Sent: Tuesday, October 30, 2018 11:49 AM
> >> To: Jiri Kosina <jikos@xxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>; Tom Lendacky
> >> <thomas.lendacky@xxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Peter
> >> Zijlstra <peterz@xxxxxxxxxxxxx>; Josh Poimboeuf <jpoimboe@xxxxxxxxxx>;
> >> Andrea Arcangeli <aarcange@xxxxxxxxxx>; David Woodhouse
> >> <dwmw@xxxxxxxxxxxx>; Andi Kleen <ak@xxxxxxxxxxxxxxx>; Hansen, Dave
> >> <dave.hansen@xxxxxxxxx>; Schaufler, Casey <casey.schaufler@xxxxxxxxx>;
> >> Mallick, Asit K <asit.k.mallick@xxxxxxxxx>; Arjan van de Ven
> >> <arjan@xxxxxxxxxxxxxxx>; Jon Masters <jcm@xxxxxxxxxx>; Waiman Long
> >> <longman9394@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx
> >> Subject: [Patch v4 16/18] x86/speculation: Enable STIBP to protect security
> >> sensitive tasks
> >>
> >> Enable STIBP defense on high security tasks.
> >>
> >> For normal tasks, STIBP is unused so they are not impacted by overhead
> >> from STIBP in lite protection mode.
> >>
> >> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >> ---
> >>  arch/x86/kernel/cpu/bugs.c | 33 +++++++++++++++++++++++++++++++++
> >>  1 file changed, 33 insertions(+)
> >>
> >> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> >> index 54f4675..b402b96 100644
> >> --- a/arch/x86/kernel/cpu/bugs.c
> >> +++ b/arch/x86/kernel/cpu/bugs.c
> >> @@ -14,6 +14,8 @@
> >>  #include <linux/module.h>
> >>  #include <linux/nospec.h>
> >>  #include <linux/prctl.h>
> >> +#include <linux/sched/coredump.h>
> >> +#include <linux/security.h>
> >>
> >>  #include <asm/spec-ctrl.h>
> >>  #include <asm/cmdline.h>
> >> @@ -770,6 +772,37 @@ static int ssb_prctl_set(struct task_struct *task,
> >> unsigned long ctrl)
> >>  	return 0;
> >>  }
> >>
> >> +static void set_task_stibp(struct task_struct *tsk, bool stibp_on)
> >> +{
> >> +	bool update = false;
> >> +
> >> +	if (!static_branch_unlikely(&spectre_v2_app_lite))
> >> +		return;
> >> +
> >> +	if (stibp_on)
> >> +		update = !test_and_set_tsk_thread_flag(tsk, TIF_STIBP);
> >> +	else
> >> +		update = test_and_clear_tsk_thread_flag(tsk, TIF_STIBP);
> >> +
> >> +	if (!update)
> >> +		return;
> >> +
> >> +	if (tsk == current)
> >> +		speculation_ctrl_update_current();
> >> +}
> >> +
> >> +void arch_set_security(struct task_struct *tsk, unsigned int value)
> >
> > In this context "security" isn't descriptive. arch_set_stibp_defenses()
> > would be better.
> 
> A more generic name decoupled from STIBP will be preferable.  There
> can other kind of security defenses to be erected in
> the future.
> 
> Perhaps arch_set_mitigation?

Better. On the other hand, adding function call layers just in case leads
to cascades of functions that do nothing but call other functions, and that
makes code hard to understand. I would leave generalization for the 2nd
person who wants to add mitigations. 

> 
> Thanks.
> 
> Tim
> 
> >
> > Since "value" should only ever have one of two values, and those
> > map directly to "true" or "false" this should be a bool, making the
> > code trivial:
> >
> > void arch_set_stibp_defenses(struct task_struct *task, bool stibp)
> > {
> > 	set_task_stibp(task, stibp);
> > }
> >
> > Or perhaps arch_set_security() should go away, and the calling
> > code would call set_task_stibp() directly. Unless there is some compelling
> > reason for the abstractions.
> >
> >> +{
> >> +	if (value > SECURITY_HIGH)
> >> +		return;
> >> +
> >> +	/* Update STIBP defenses */
> >> +	if (value == SECURITY_HIGH)
> >> +		set_task_stibp(tsk, true);
> >> +	else
> >> +		set_task_stibp(tsk, false);
> >> +}
> >> +
> >>  int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
> >>  			     unsigned long ctrl)
> >>  {
> >> --
> >> 2.9.4
> >