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