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

[Tim Chen]

On 11/07/2018 10:33 AM, Waiman Long wrote:
> 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?

Ah yes, should be t.  t==current is checked in set_task_stibp.

Tim

> 
>> +		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
>