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

[Tim Chen]

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?

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
>