Re: [Patch v4 17/18] x86/speculation: Update SPEC_CTRL MSRs of remote CPUs

[Waiman Long <longman@redhat.com>]

On 11/06/2018 07:18 PM, Tim Chen wrote:
> Thomas,
>
>>>> 2) Add _TIF_UPDATE_SPEC_CTRL to the SYSCALL_EXIT_WORK_FLAGS and handle it
>>>>    in the slow work path.
>>> There can be tasks that don't do any syscalls, and it seems like we can
>>> have MSRs getting out of sync?
>> Setting the TIF flag directly in a remote task is wrong. It needs to be
>> handled when the _TIF_UPDATE_SPEC_CTRL is evaluated, i.e. the information
>> needs to be stored process wide e.g. in task->mm.
>>
>> But yes, if the remote task runs in user space forever, it won't
>> help. Though the point is that dumpable is usually set when the process
>> starts, so it's probably mostly a theoretical issue.
>>
> I took a crack to implement what you suggested to update
> remote task's flag and remote SPEC_CTRL MSR on the syscall exit slow path.
>
> This looks reasobale?
>
> Tim
>
>
> ------------
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 3b2490b..614594a 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -216,7 +216,7 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
>  
>  #define SYSCALL_EXIT_WORK_FLAGS				\
>  	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |	\
> -	 _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT)
> +	 _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT | _TIF_UPDATE_SPEC_CTRL)
>  
>  static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
>  {
> @@ -227,6 +227,8 @@ static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
>  	if (cached_flags & _TIF_SYSCALL_TRACEPOINT)
>  		trace_sys_exit(regs, regs->ax);
>  
> +	if (cached_flags & _TIF_UPDATE_SPEC_CTRL)
> +		spec_ctrl_do_pending_update();
>  	/*
>  	 * If TIF_SYSCALL_EMU is set, we only get here because of
>  	 * TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP).
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index c59a6c4..f124597 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -276,6 +276,8 @@ static inline void indirect_branch_prediction_barrier(void)
>  	alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
>  }
>  
> +void spec_ctrl_do_pending_update(void);
> +
>  /* The Intel SPEC CTRL MSR base value cache */
>  extern u64 x86_spec_ctrl_base;
>  
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 4f6a7a9..b78db59 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -97,6 +97,7 @@ struct thread_info {
>  #define TIF_USER_RETURN_NOTIFY	14	/* Notify kernel of userspace return */
>  #define TIF_PATCH_PENDING	15	/* Pending live patching update */
>  #define TIF_FSCHECK		16	/* Check FS is USER_DS on return */
> +#define TIF_UPDATE_SPEC_CTRL	17	/* Pending update of speculation control */
>  
>  /* Task status */
>  #define TIF_UPROBE		18	/* Breakpointed or singlestepping */
> @@ -131,6 +132,7 @@ struct thread_info {
>  #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
>  #define _TIF_PATCH_PENDING	(1 << TIF_PATCH_PENDING)
>  #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
> +#define _TIF_UPDATE_SPEC_CTRL	(1 << TIF_UPDATE_SPEC_CTRL)
>  
>  #define _TIF_UPROBE		(1 << TIF_UPROBE)
>  #define _TIF_MEMDIE		(1 << TIF_MEMDIE)
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 4c15c86..d82d3f8 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/sched/signal.h>
>  
>  #include <asm/spec-ctrl.h>
>  #include <asm/cmdline.h>
> @@ -770,6 +772,69 @@ 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 (stibp_on)
> +		update = !test_and_set_tsk_thread_flag(tsk, TIF_STIBP);
> +	else
> +		update = test_and_clear_tsk_thread_flag(tsk, TIF_STIBP);
> +
> +	if (tsk == current && update)
> +		speculation_ctrl_update_current();
> +}
> +
> +void spec_ctrl_do_pending_update(void)
> +{
> +	if (!static_branch_unlikely(&spectre_v2_app_lite))
> +		return;
> +
> +	if (!current->mm)
> +		return;
> +
> +	if (get_dumpable(current->mm) != SUID_DUMP_USER)
> +		set_tsk_thread_flag(current, TIF_STIBP);
> +	else
> +		clear_tsk_thread_flag(current, TIF_STIBP);
> +
> +	clear_tsk_thread_flag(current, TIF_UPDATE_SPEC_CTRL);
> +	speculation_ctrl_update_current();
> +}
> + 
> +int arch_update_spec_ctrl_restriction(struct task_struct *task)
> +{
> +	unsigned long flags;
> +	struct task_struct *t;
> +	bool stibp_on = false;
> +
> +	if (!static_branch_unlikely(&spectre_v2_app_lite))
> +		return 0;
> +
> +	if (!task->mm)
> +		return -EINVAL;
> +
> +	if (!lock_task_sighand(task, &flags))
> +		return -ESRCH;
> +
> +	if (get_dumpable(task->mm) != SUID_DUMP_USER)
> +		stibp_on = true;
> +
> +	for_each_thread(task, t) {
> +		if (task_cpu(task) == smp_processor_id())
> +			set_task_stibp(task, stibp_on);

I think "t" is the iterator, not "task". BTW, a thread is on the same
CPU doesn't mean it is running. Should you just check "(t == current)" here?

> +		else if (test_tsk_thread_flag(task, TIF_STIBP) != stibp_on) 
> +			set_tsk_thread_flag(task, TIF_UPDATE_SPEC_CTRL);
> +	}
> +
> +	unlock_task_sighand(task, &flags);
> +	return 0;
> +}
> +
>  int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
>  			     unsigned long ctrl)
>  {

Cheers,
Longman