lkml.org 
[lkml]   [2018]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [patch 20/24] x86/speculation: Split out TIF update
    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

    \
     
     \ /
      Last update: 2018-11-27 23:07    [W:5.392 / U:0.000 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site