On Tue, 27 Nov 2018, Thomas Gleixner wrote: > > 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..6c4fcef52b19 100644 > > --- a/arch/x86/kernel/process.c > > +++ b/arch/x86/kernel/process.c > > @@ -474,6 +474,21 @@ void __switch_to_xtra(struct task_struct *prev_p, > > struct task_struct *next_p) > > > > tifn = READ_ONCE(task_thread_info(next_p)->flags); > > tifp = READ_ONCE(task_thread_info(prev_p)->flags); > > + > > + /* > > + * SECCOMP tasks might have had their spec_ctrl flags updated during > > + * runtime from a different CPU. > > + * > > + * When switching to such a task, populate thread flags with the ones > > + * that have been temporarily saved in spec_flags by > > task_update_spec_tif() > > + * in order to make sure MSR value is always kept up to date. > > + * > > + * SECCOMP tasks never disable the mitigation for other threads, only > > enable. > > + */ > > + if (IS_ENABLED(CONFIG_SECCOMP) && > > + test_and_clear_tsk_thread_flag(next_p, TIF_SPEC_UPDATE)) > > + tifp |= READ_ONCE(task_thread_info(next_p)->spec_flags); > > And how does that get folded into task_thread_info(next_p)->flags for the > next context switch?
Does it really have to? We need this special handling only if the next task has TIF_SPEC_UPDATE set, which is one-off event globally (when seccomp marks all its threads so due to seccomp filter change), and once all the TIF_SPEC_UPDATE tasks schedule at least once, we're in a consistent state again and don't need this, as every running task will then have its TIF consistent with MSR value. Thanks, -- Jiri Kosina SUSE Labs
