linux-kernel - Re: [PATCH v11 10/12] ptrace,seccomp: Add PTRACE

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Message-ID: <CABqD9hZ3apFGi+BmX5UNgz=NJUbTigj6acDKG=wu9rrL0wN1Ww@mail.gmail.com>
Date:	Tue, 28 Feb 2012 12:34:43 -0600
From:	Will Drewry <wad@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	peterz@...radead.org, rdunlap@...otime.net, mcgrathr@...omium.org,
	tglx@...utronix.de, luto@....edu, eparis@...hat.com,
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com,
	indan@....nu, pmoore@...hat.com, akpm@...ux-foundation.org,
	corbet@....net, eric.dumazet@...il.com, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com, keescook@...omium.org,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH v11 10/12] ptrace,seccomp: Add PTRACE_SECCOMP support

On Tue, Feb 28, 2012 at 11:04 AM, Will Drewry <wad@...omium.org> wrote:
> On Tue, Feb 28, 2012 at 10:43 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 02/27, Will Drewry wrote:
>>> On Mon, Feb 27, 2012 at 11:54 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>>> > On 02/24, Will Drewry wrote:
>>> >>
>>> >>  arch/Kconfig              |    1 +
>>> >>  include/linux/ptrace.h    |    7 +++++--
>>> >>  include/linux/seccomp.h   |    4 +++-
>>> >>  include/linux/tracehook.h |    6 ++++++
>>> >>  kernel/ptrace.c           |    4 ++++
>>> >>  kernel/seccomp.c          |   18 ++++++++++++++++++
>>> >
>>> > FYI, this conflicts with the changes -mm tree.
>>> >
>>> > The changes in ptrace.* confict with Denys's
>>> > "ptrace: simplify PTRACE_foo constants and PTRACE_SETOPTIONS code"
>>> >
>>> > The change in tracehook.h conflicts with
>>> > "ptrace: the killed tracee should not enter the syscall"
>>>
>>> What's the best way to reconcile this in this day and age?
>>
>> Of course I'd prefer if you make this change on top of Denys's patch ;)
>>
>> Besides, if you agree with PTRACE_EVENT_SECCOMP/PTRACE_O_SECCOMP you
>> need only one trivial change in ptrace.h.
>
> I think that works quite well :)
>
>>> I don't see
>>> these in kernel-next yet and I can't tell if there is a public -mm
>>> anywhere anymore.
>>
>> Strange... I didn't check, but every patch in
>> http://marc.info/?l=linux-mm-commits has this note:
>>
>>        The -mm tree is included into linux-next and is updated
>>        there every 3-4 working days
>
> It appears to have been pulled in ~8 hours ago.  I'm rebasing to next now.
>
>>> >> --- a/kernel/seccomp.c
>>> >> +++ b/kernel/seccomp.c
>>> >> @@ -354,6 +354,24 @@ int __secure_computing_int(int this_syscall)
>>> >>                       seccomp_send_sigsys(this_syscall, reason_code);
>>> >>                       return -1;
>>> >>               }
>>> >> +             case SECCOMP_RET_TRACE: {
>>> >> +                     int ret;
>>> >> +                     struct pt_regs *regs = task_pt_regs(current);
>>> >> +                     if (!(test_tsk_thread_flag(current, TIF_SYSCALL_TRACE)) ||
>>> >> +                         !(current->ptrace & PT_TRACE_SECCOMP))
>>> >> +                             return -1;
>>> >> +                     /*
>>> >> +                      * PT_TRACE_SECCOMP and seccomp.trace indicate whether
>>> >> +                      * tracehook_report_syscall_entry needs to signal the
>>> >> +                      * tracer.  This avoids race conditions in hand off and
>>> >> +                      * the requirement for TIF_SYSCALL_TRACE ensures that
>>> >> +                      * we are in the syscall slow path.
>>> >> +                      */
>>> >> +                     current->seccomp.trace = 1;
>>> >> +                     ret = tracehook_report_syscall_entry(regs);
>>> >> +                     current->seccomp.trace = 0;
>>> >> +                     return ret;
>>> >
>>> > To be honest, this interface looks a bit strange to me...
>>> >
>>> > Once again, sorry if this was already discussed. But perhaps it would
>>> > be better to introduce PTRACE_EVENT_SECCOMP/PTRACE_O_SECCOMP instead?
>>> >
>>> > SECCOMP_RET_TRACE: could simply do ptrace_event(PTRACE_EVENT_SECCOMP)
>>> > unconditionaly. The tracer can set the option and do PTRACE_CONT if it
>>> > doesn't want the system call notifications.
>>>
>>> Works for me - this also gets rid of the extra int for brief state
>>> tracking. I'll switch over to that in the next rev.
>>
>> Great. In this case this patch becomes really trivial. Just 2 defines
>> in ptrace.h and the unconditional ptrace_event() under SECCOMP_RET_TRACE.

hrm the only snag is that I can't then rely on TIF_SYSCALL_TRACE to
ensure seccomp is in the slow-path.  Right now, on x86, seccomp is
slow-path, but it doesn't have to be to have the syscall and args.
However, for ptrace to behavior properly, I believed it did need to be
in the slow path.  If SECCOMP_RET_TRACE doesn't rely on
PTRACE_SYSCALL, then it introduces a need for seccomp to always be in
the slow path or to flag (somehow) when it needs slow path.

Any suggestions there?

Thanks!
will
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