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

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Feb 5, 2015 at 1:40 PM, Dmitry V. Levin <ldv@altlinux.org> wrote:
> 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 <ldv@altlinux.org> 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 <luto@amacapital.net>
>> >> ---
>> >>  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.
>
>

Hasn't it always been this way?

I admit that I kind of wish this worked the other way -- that is, I
think it would be nice to have a mode in which ptrace runs before
seccomp, which would close the ptrace hole (where ptrace can do things
that seccomp would disallow) and maybe have more comprehensible
results.

--Andy

> --
> ldv



-- 
Andy Lutomirski
AMA Capital Management, LLC