Re: Using ftrace/perf as a basis for generic seccomp

[Masami Hiramatsu]

Hi Eric,

(2011/02/01 23:58), Eric Paris wrote:
> On Wed, Jan 12, 2011 at 4:28 PM, Eric Paris <eparis@xxxxxxxxxx> wrote:
>> Some time ago Adam posted a patch to allow for a generic seccomp
>> implementation (unlike the current seccomp where your choice is all
>> syscalls or only read, write, sigreturn, and exit) which got little
>> traction and it was suggested he instead do the same thing somehow using
>> the tracing code:
>> http://thread.gmane.org/gmane.linux.kernel/833556

Hm, interesting idea :)
But why would you like to use tracing code? just for hooking?

>> The actual method that this could be achieved was apparently left as an
>> exercise for the reader.  Since I'd like to do something similar (and
>> actually basically reimplemented Adam's code before I found this thread)
>> I guess that makes me the reader.  I've never touched
>> perf/ftrace/whatever so I'm not even knowledgeably enough to ask good
>> questions so please, try to talk to me like a 2 year old.

OK, I'll try to explain;

Ftrace/perf syscall event tracing is based on syscall tracepoints
(sys_enter and sys_exit) which are implemented as a special hook
requiring TIF_SYSCALL_TRACEPOINT.

--<arch/x86/kernel/ptrace.c>--
asmregparm long syscall_trace_enter(struct pt_regs *regs)
{
        long ret = 0;
[...]
        /* do the secure computing check first */
        secure_computing(regs->orig_ax);		<-- secomp!

        if (unlikely(test_thread_flag(TIF_SYSCALL_EMU)))
                ret = -1L;

        if ((ret || test_thread_flag(TIF_SYSCALL_TRACE)) &&
            tracehook_report_syscall_entry(regs))
                ret = -1L;

        if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
                trace_sys_enter(regs, regs->orig_ax);	<--here!
----

All syscalls issued by threads which has TIF_SYSCALL_TRACEPOINT
kick trace_sys_enter() tracepoint, and then the tracepoint calls
ftrace handler or perf handler.

And this tracepoint is not only for ftrace/perf, but also you
can use it directly via register_trace_sys_enter() (the tracepoint
can be shared among several handlers). If you just want to hook
the syscall entry, I recommend that instead of modifying ftrace/perf.
See kernel/trace/trace_syscalls.c, Documentation/trace/tracepoints.txt
and samples/tracepoints/ for details.

However, I think here is an ordering problem. As you can see, secomp
hook is done before these hooks, that might cause a problem because
tracehook_report_syscall_entry(), which is also done before tracepoint,
is used by ptrace().
This means that someone can hook into an unsafe syscall via debugger.

So, finally, I think you'd better expand secure_computing() hook, or
introduce more generic hook-point.

Thank you,

-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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