On Thu, Feb 5, 2015 at 1:52 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Feb 5, 2015 at 1:40 PM, Dmitry V. Levin <ldv@xxxxxxxxxxxx> 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@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.)
>>
>> 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?
As far as I know, yes, it's always been this way. The point is to the
skip the syscall, which is what the -1 signals. Userspace then reads
back the errno.
> 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.
I prefer keeping the seccomp attack surface as tiny as possible. I
would not like to ptrace happening before seccomp.
-Kees
--
Kees Cook
Chrome OS Security
