On Tue, 27 Nov 2018, Jiri Kosina wrote: > On Mon, 26 Nov 2018, Thomas Gleixner wrote: > > How about the minimalistic aproach below? (only compile tested so far, > applies on top of your latest WIP.x86/pti branch). The downside of course > is wasting another TIF bit.
We need to waste another TIF bit in any case. > * > * This can only happen for SECCOMP mitigation. For PRCTL it's > * always the current task. > + * > + * If we are updating non-current task, set a flag for it to always > + * perform the MSR sync on a first context switch, to make sure > + * the TIF_SPEC_IB above is not out of sync with the MSR value during > + * task's runtime. > */ > if (tsk == current && update) > speculation_ctrl_update_current(); > + else > + set_tsk_thread_flag(tsk, TIF_SPEC_UPDATE); > + > } > > static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl) > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c > index 3f5e351bdd37..78208234e63e 100644 > --- a/arch/x86/kernel/process.c > +++ b/arch/x86/kernel/process.c > @@ -449,8 +449,20 @@ static __always_inline void > __speculation_ctrl_update(unsigned long tifp, > * otherwise avoid the MSR write. > */ > if (IS_ENABLED(CONFIG_SMP) && > - static_branch_unlikely(&switch_to_cond_stibp)) > + static_branch_unlikely(&switch_to_cond_stibp)) { > updmsr |= !!(tif_diff & _TIF_SPEC_IB); > + /* > + * We need to update the MSR if remote task did set > + * TIF_SPEC_UPDATE on us, and therefore MSR value and > + * the TIF_SPEC_IB values might be out of sync. > + * > + * This can only happen if seccomp task has updated > + * one of its remote threads. > + */ > + if (IS_ENABLED(CONFIG_SECCOMP) && !updmsr && > + (tifn & TIF_SPEC_UPDATE)) > + updmsr = true; > + } > > if (updmsr) > spec_ctrl_update_msr(tifn); > @@ -496,6 +508,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct > task_struct *next_p) > set_cpuid_faulting(!!(tifn & _TIF_NOCPUID)); > > __speculation_ctrl_update(tifp, tifn); > + if (IS_ENABLED(CONFIG_SECCOMP)) > + clear_tsk_thread_flag(next_p, TIF_SPEC_UPDATE); That's racy and does not prevent the situation because the TIF flags are updated befor the UPDATE bit is set. So __speculation_ctrl_update() might see the new bits, but not TIF_SPEC_UPDATE. You really need shadow storage to avoid that. Thanks, tglx