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
