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

[Kees Cook]

On Thu, Feb 5, 2015 at 1:19 PM, Dmitry V. Levin <ldv@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> ---
>>  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.)

-Kees

-- 
Kees Cook
Chrome OS Security
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