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

[cc: Eric Biederman]

On Fri, Jul 25, 2014 at 10:10 AM, Kees Cook [off-list ref] wrote:

On Fri, Jul 25, 2014 at 8:59 AM, Andy Lutomirski [off-list ref] wrote:

quoted

On Jul 25, 2014 6:48 AM, "David Drysdale" [off-list ref] wrote:

quoted

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 <redacted>
---
 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.

quoted

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).

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.

--Andy

-Kees

--
Kees Cook
Chrome OS Security

-- 
Andy Lutomirski
AMA Capital Management, LLC