Re: [PATCH 2/2] x86/entry: Do enter_from_user_mode with IRQs off

[Andy Lutomirski]

On Mar 8, 2016 5:46 AM, "Borislav Petkov" <bp@xxxxxxxxx> wrote:
>
> On Sun, Mar 06, 2016 at 06:30:07PM -0800, Andy Lutomirski wrote:
> > Now that slow-path syscalls always enter C before enabling
> > interrupts, it's straightforward to do enter_from_user_mode before
> > enabling interrupts rather than doing it as part of entry tracing.
> >
> > With this change, we should finally be able to retire
> > exception_enter.
> >
> > This will also enable optimizations based on knowing that we never
> > change context tracking state with interrupts on.
> >
> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> > ---
> >  arch/x86/entry/common.c            | 39 ++++++++++++++------------------------
> >  arch/x86/include/asm/thread_info.h |  5 ++++-
> >  2 files changed, 18 insertions(+), 26 deletions(-)
> >
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index 9ef3258320ec..8e8ca0eb36be 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -38,14 +38,17 @@ static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
> >       return (struct thread_info *)(top_of_stack - THREAD_SIZE);
> >  }
> >
> > -#ifdef CONFIG_CONTEXT_TRACKING
> > +#ifndef CONFIG_CONTEXT_TRACKING
> > +static
> > +#else
> > +__visible
> > +#endif
> >  /* Called on entry from user mode with IRQs off. */
> > -__visible void enter_from_user_mode(void)
> > +void enter_from_user_mode(void)
> >  {
> >       CT_WARN_ON(ct_state() != CONTEXT_USER);
> >       user_exit();
> >  }
> > -#endif
>
> I'm guessing you're relying on the compiler to optimize out everything
> in the !CONFIG_CONTEXT_TRACKING case, and it seems to do that here.
>
> However, I would do the regular thing we do for cases like that. It is
> much easier and faster to read because we do use it everywhere:
>
> #ifdef CONFIG_CONTEXT_TRACKING
> /* Called on entry from user mode with IRQs off. */
> __visible void enter_from_user_mode(void)
> {
>         CT_WARN_ON(ct_state() != CONTEXT_USER);
>         user_exit();
> }
> #else
> #define enter_from_user_mode() do { } while (0)
> #endif
>

Will do, after I test to make sure it builds.

>
> >
> >  static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> >  {
> > @@ -85,17 +88,6 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
> >
> >       work = ACCESS_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY;
> >
> > -#ifdef CONFIG_CONTEXT_TRACKING
> > -     /*
> > -      * If TIF_NOHZ is set, we are required to call user_exit() before
> > -      * doing anything that could touch RCU.
> > -      */
> > -     if (work & _TIF_NOHZ) {
> > -             enter_from_user_mode();
> > -             work &= ~_TIF_NOHZ;
> > -     }
> > -#endif
> > -
> >  #ifdef CONFIG_SECCOMP
> >       /*
> >        * Do seccomp first -- it should minimize exposure of other
> > @@ -354,6 +346,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
> >       struct thread_info *ti = pt_regs_to_thread_info(regs);
> >       unsigned long nr = regs->orig_ax;
> >
> > +     enter_from_user_mode();
> >       local_irq_enable();
> >
> >       if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
> > @@ -376,9 +369,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
> >
> >  #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
> >  /*
> > - * Does a 32-bit syscall.  Called with IRQs on and does all entry and
> > - * exit work and returns with IRQs off.  This function is extremely hot
> > - * in workloads that use it, and it's usually called from
> > + * Does a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.  Does
> > + * all entry and exit work and returns with IRQs off.  This function is
> > + * extremely hot in workloads that use it, and it's usually called from
> >   * do_fast_syscall_32, so forcibly inline it to improve performance.
> >   */
> >  static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
> > @@ -419,6 +412,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
> >  /* Handles int $0x80 */
> >  __visible void do_int80_syscall_32(struct pt_regs *regs)
> >  {
> > +     enter_from_user_mode();
> >       local_irq_enable();
> >       do_syscall_32_irqs_on(regs);
> >  }
> > @@ -441,11 +435,9 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
> >        */
> >       regs->ip = landing_pad;
> >
> > -     /*
> > -      * Fetch EBP from where the vDSO stashed it.
> > -      *
> > -      * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
> > -      */
> > +     enter_from_user_mode();
> > +
> > +     /* Fetch EBP from where the vDSO stashed it. */
> >       local_irq_enable();
>
> Comment above should be here.

Indeed.

--Andy