Re: [PATCH 11/11] seccomp: Add tgid and tid into seccomp_data

[Kees Cook <keescook@chromium.org>]

On Fri, Jul 25, 2014 at 8:59 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Jul 25, 2014 6:48 AM, "David Drysdale" <drysdale@google.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@google.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?

-Kees

-- 
Kees Cook
Chrome OS Security