On Thu, Feb 5, 2015 at 1:52 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Thu, Feb 5, 2015 at 1:40 PM, Dmitry V. Levin <l...@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 <l...@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 <l...@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?
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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/