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 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.

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