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SubjectRE: [Patch v4 16/18] x86/speculation: Enable STIBP to protect security sensitive tasks
Date
> -----Original Message-----
> From: Tim Chen [mailto:tim.c.chen@linux.intel.com]
> Sent: Tuesday, October 30, 2018 11:49 AM
> To: Jiri Kosina <jikos@kernel.org>; Thomas Gleixner <tglx@linutronix.de>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>; Tom Lendacky
> <thomas.lendacky@amd.com>; Ingo Molnar <mingo@redhat.com>; Peter
> Zijlstra <peterz@infradead.org>; Josh Poimboeuf <jpoimboe@redhat.com>;
> Andrea Arcangeli <aarcange@redhat.com>; David Woodhouse
> <dwmw@amazon.co.uk>; Andi Kleen <ak@linux.intel.com>; Hansen, Dave
> <dave.hansen@intel.com>; Schaufler, Casey <casey.schaufler@intel.com>;
> Mallick, Asit K <asit.k.mallick@intel.com>; Arjan van de Ven
> <arjan@linux.intel.com>; Jon Masters <jcm@redhat.com>; Waiman Long
> <longman9394@gmail.com>; linux-kernel@vger.kernel.org; x86@kernel.org
> 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@linux.intel.com>
> ---
> 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

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