linux-kernel - Re: [PATCH 11/11] seccomp: Add tgid and tid into seccomp

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Date:	Fri, 25 Jul 2014 10:38:14 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Julien Tinnes <jln@...gle.com>,
	David Drysdale <drysdale@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paolo Bonzini <pbonzini@...hat.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	James Morris <james.l.morris@...cle.com>,
	Linux API <linux-api@...r.kernel.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/11] seccomp: Add tgid and tid into seccomp_data

On Fri, Jul 25, 2014 at 10:18 AM, Andy Lutomirski <luto@...capital.net> wrote:
> [cc: Eric Biederman]
>
> On Fri, Jul 25, 2014 at 10:10 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Fri, Jul 25, 2014 at 8:59 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Jul 25, 2014 6:48 AM, "David Drysdale" <drysdale@...gle.com> wrote:
>>>>
>>>> Add the current thread and thread group IDs into the data
>>>> available for seccomp-bpf programs to work on.  This allows
>>>> installation of filters that police syscalls based on thread
>>>> or process ID, e.g. tgkill(2)/kill(2)/prctl(2).
>>>>
>>>> Signed-off-by: David Drysdale <drysdale@...gle.com>
>>>> ---
>>>>  include/uapi/linux/seccomp.h | 10 ++++++++++
>>>>  kernel/seccomp.c             |  2 ++
>>>>  2 files changed, 12 insertions(+)
>>>>
>>>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>>>> index ac2dc9f72973..b88370d6f6ca 100644
>>>> --- a/include/uapi/linux/seccomp.h
>>>> +++ b/include/uapi/linux/seccomp.h
>>>> @@ -36,12 +36,22 @@
>>>>   * @instruction_pointer: at the time of the system call.
>>>>   * @args: up to 6 system call arguments always stored as 64-bit values
>>>>   *        regardless of the architecture.
>>>> + * @tgid: thread group ID of the thread executing the BPF program.
>>>> + * @tid: thread ID of the thread executing the BPF program.
>>>> + * The SECCOMP_DATA_TID_PRESENT macro indicates the presence of the
>>>> + * tgid and tid fields; user programs may use this macro to conditionally
>>>> + * compile code against older versions of the kernel.  Note also that
>>>> + * BPF programs should cope with the absence of these fields by testing
>>>> + * the length of data available.
>>>>   */
>>>>  struct seccomp_data {
>>>>         int nr;
>>>>         __u32 arch;
>>>>         __u64 instruction_pointer;
>>>>         __u64 args[6];
>>>> +       __u32 tgid;
>>>> +       __u32 tid;
>>>>  };
>>>> +#define SECCOMP_DATA_TID_PRESENT       1
>>>>
>>>>  #endif /* _UAPI_LINUX_SECCOMP_H */
>>>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>>>> index 301bbc24739c..dd5146f15d6d 100644
>>>> --- a/kernel/seccomp.c
>>>> +++ b/kernel/seccomp.c
>>>> @@ -80,6 +80,8 @@ static void populate_seccomp_data(struct seccomp_data *sd)
>>>>         sd->args[4] = args[4];
>>>>         sd->args[5] = args[5];
>>>>         sd->instruction_pointer = KSTK_EIP(task);
>>>> +       sd->tgid = task_tgid_vnr(current);
>>>> +       sd->tid = task_pid_vnr(current);
>>>>  }
>>>
>>> This is, IMO, problematic.  These should probably be relative to the
>>> filter creator, not the filtered task.  This will also hurt
>>> performance.
>>
>> Yeah, we can't change the seccomp_data structure without a lot of
>> care, and tgid/tid really should be encoded in the filter. However, it
>> is tricky in the forking case.
>>
>>>
>>> What's the use case?  Can it be better achieved with a new eBPF function?
>>
>> Julien had been wanting something like this too (though he'd suggested
>> it via prctl): limit the signal functions to "self" only. I wonder if
>> adding a prctl like done for O_BENEATH could work for signal sending?
>>
>
>
> Can we do one better and add a flag to prevent any non-self pid
> lookups?  This might actually be easy on top of the pid namespace work
> (e.g. we could change the way that find_task_by_vpid works).

Ooh, that would be extremely interesting, yes. Kind of an extreme form
of pid namespace without actually being a namespace.

> It's far from just being signals.  There's access_process_vm, ptrace,
> all the signal functions, clock_gettime (see CPUCLOCK_PID -- yes, this
> is ridiculous), and probably some others that I've forgotten about or
> never noticed in the first place.

Yeah, that would be very interesting.

-Kees

-- 
Kees Cook
Chrome OS Security
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