Subject: [PATCH v5 3/5] x86: Split syscall_trace_enter into two phases

From: [email-masked] (Dmitry V. Levin)
Date: Fri, 6 Feb 2015 00:40:27 +0300

On Thu, Feb 05, 2015 at 01:27:16PM -0800, Kees Cook wrote:
> On Thu, Feb 5, 2015 at 1:19 PM, Dmitry V. Levin <[email-masked]> wrote:
> > Hi,
> >
> > On Fri, Sep 05, 2014 at 03:13:54PM -0700, Andy Lutomirski wrote:
> >> This splits syscall_trace_enter into syscall_trace_enter_phase1 and
> >> syscall_trace_enter_phase2.  Only phase 2 has full pt_regs, and only
> >> phase 2 is permitted to modify any of pt_regs except for orig_ax.
> >
> > This breaks ptrace, see below.
> >
> >> The intent is that phase 1 can be called from the syscall fast path.
> >>
> >> In this implementation, phase1 can handle any combination of
> >> TIF_NOHZ (RCU context tracking), TIF_SECCOMP, and TIF_SYSCALL_AUDIT,
> >> unless seccomp requests a ptrace event, in which case phase2 is
> >> forced.
> >>
> >> In principle, this could yield a big speedup for TIF_NOHZ as well as
> >> for TIF_SECCOMP if syscall exit work were similarly split up.
> >>
> >> Signed-off-by: Andy Lutomirski <[email-masked]>
> >> ---
> >>  arch/x86/include/asm/ptrace.h |   5 ++
> >>  arch/x86/kernel/ptrace.c      | 157 +++++++++++++++++++++++++++++++++++-------
> >>  2 files changed, 138 insertions(+), 24 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> >> index 6205f0c434db..86fc2bb82287 100644
> >> --- a/arch/x86/include/asm/ptrace.h
> >> +++ b/arch/x86/include/asm/ptrace.h
> >> @@ -75,6 +75,11 @@ convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs);
> >>  extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
> >>                        int error_code, int si_code);
> >>
> >> +
> >> +extern unsigned long syscall_trace_enter_phase1(struct pt_regs *, u32 arch);
> >> +extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch,
> >> +                                    unsigned long phase1_result);
> >> +
> >>  extern long syscall_trace_enter(struct pt_regs *);
> >>  extern void syscall_trace_leave(struct pt_regs *);
> >>
> >> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> >> index bbf338a04a5d..29576c244699 100644
> >> --- a/arch/x86/kernel/ptrace.c
> >> +++ b/arch/x86/kernel/ptrace.c
> >> @@ -1441,20 +1441,126 @@ void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
> >>       force_sig_info(SIGTRAP, &info, tsk);
> >>  }
> >>
> >> +static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> >> +{
> >> +#ifdef CONFIG_X86_64
> >> +     if (arch == AUDIT_ARCH_X86_64) {
> >> +             audit_syscall_entry(arch, regs->orig_ax, regs->di,
> >> +                                 regs->si, regs->dx, regs->r10);
> >> +     } else
> >> +#endif
> >> +     {
> >> +             audit_syscall_entry(arch, regs->orig_ax, regs->bx,
> >> +                                 regs->cx, regs->dx, regs->si);
> >> +     }
> >> +}
> >> +
> >>  /*
> >> - * We must return the syscall number to actually look up in the table.
> >> - * This can be -1L to skip running any syscall at all.
> >> + * We can return 0 to resume the syscall or anything else to go to phase
> >> + * 2.  If we resume the syscall, we need to put something appropriate in
> >> + * regs->orig_ax.
> >> + *
> >> + * NB: We don't have full pt_regs here, but regs->orig_ax and regs->ax
> >> + * are fully functional.
> >> + *
> >> + * For phase 2's benefit, our return value is:
> >> + * 0:                        resume the syscall
> >> + * 1:                        go to phase 2; no seccomp phase 2 needed
> >> + * anything else:    go to phase 2; pass return value to seccomp
> >>   */
> >> -long syscall_trace_enter(struct pt_regs *regs)
> >> +unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
> >>  {
> >> -     long ret = 0;
> >> +     unsigned long ret = 0;
> >> +     u32 work;
> >> +
> >> +     BUG_ON(regs != task_pt_regs(current));
> >> +
> >> +     work = ACCESS_ONCE(current_thread_info()->flags) &
> >> +             _TIF_WORK_SYSCALL_ENTRY;
> >>
> >>       /*
> >>        * If TIF_NOHZ is set, we are required to call user_exit() before
> >>        * doing anything that could touch RCU.
> >>        */
> >> -     if (test_thread_flag(TIF_NOHZ))
> >> +     if (work & _TIF_NOHZ) {
> >>               user_exit();
> >> +             work &= ~TIF_NOHZ;
> >> +     }
> >> +
> >> +#ifdef CONFIG_SECCOMP
> >> +     /*
> >> +      * Do seccomp first -- it should minimize exposure of other
> >> +      * code, and keeping seccomp fast is probably more valuable
> >> +      * than the rest of this.
> >> +      */
> >> +     if (work & _TIF_SECCOMP) {
> >> +             struct seccomp_data sd;
> >> +
> >> +             sd.arch = arch;
> >> +             sd.nr = regs->orig_ax;
> >> +             sd.instruction_pointer = regs->ip;
> >> +#ifdef CONFIG_X86_64
> >> +             if (arch == AUDIT_ARCH_X86_64) {
> >> +                     sd.args[0] = regs->di;
> >> +                     sd.args[1] = regs->si;
> >> +                     sd.args[2] = regs->dx;
> >> +                     sd.args[3] = regs->r10;
> >> +                     sd.args[4] = regs->r8;
> >> +                     sd.args[5] = regs->r9;
> >> +             } else
> >> +#endif
> >> +             {
> >> +                     sd.args[0] = regs->bx;
> >> +                     sd.args[1] = regs->cx;
> >> +                     sd.args[2] = regs->dx;
> >> +                     sd.args[3] = regs->si;
> >> +                     sd.args[4] = regs->di;
> >> +                     sd.args[5] = regs->bp;
> >> +             }
> >> +
> >> +             BUILD_BUG_ON(SECCOMP_PHASE1_OK != 0);
> >> +             BUILD_BUG_ON(SECCOMP_PHASE1_SKIP != 1);
> >> +
> >> +             ret = seccomp_phase1(&sd);
> >> +             if (ret == SECCOMP_PHASE1_SKIP) {
> >> +                     regs->orig_ax = -1;
> >
> > How the tracer is expected to get the correct syscall number after that?
> 
> There shouldn't be a tracer if a skip is encountered. (A seccomp skip
> would skip ptrace.) This behavior hasn't changed, but maybe I don't
> see what you mean? (I haven't encountered any problems with syscall
> tracing as a result of these changes.)

SECCOMP_RET_ERRNO leads to SECCOMP_PHASE1_SKIP, and if there is a tracer,
it will get -1 as a syscall number.

I've found this while testing a strace parser for
SECCOMP_MODE_FILTER/SECCOMP_SET_MODE_FILTER, so the problem is quite real.

-- 
ldv