[PATCH v5 3/5] x86: Split syscall_trace_enter into two phases

[Kees Cook]

On Fri, Feb 6, 2015 at 12:12 PM, Andy Lutomirski <luto at amacapital.net> wrote:
> On Fri, Feb 6, 2015 at 12:07 PM, Kees Cook <keescook at chromium.org> wrote:
>> On Fri, Feb 6, 2015 at 11:32 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>>> On Fri, Feb 6, 2015 at 11:23 AM, Kees Cook <keescook at chromium.org> wrote:
>>>> And especially since a ptracer
>>>> can change syscalls during syscall-enter-stop to any syscall it wants,
>>>> bypassing seccomp. This condition is already documented.
>>>
>>> If a ptracer (using PTRACE_SYSCALL) were to get the entry callback
>>> before seccomp, then this oddity would go away, which might be a good
>>> thing.  A ptracer could change the syscall, but seccomp would based on
>>> what the ptracer changed the syscall to.
>>
>> I want kill events to trigger immediately. I don't want to leave the
>> ptrace surface available on a SECCOMP_RET_KILL. So maybe it can be
>> seccomp phase 1, then ptrace, then seccomp phase 2? And pass more
>> information between phases to determine how things should behave
>> beyond just "skip"?
>
> I thought so too, originally, but I'm far less convinced now, for two reasons:
>
> 1. I think that a lot of filters these days use RET_ERRNO heavily, so
> this won't benefit them.
>
> 2. I'm not convinced it really reduces the attack surface for anyone.
> Unless your filter is literally "return SECCOMP_RET_KILL", then the
> seccomp-filtered task can always cause the ptracer to get a pair of
> syscall notifications.  Also, the task can send itself signals (using
> page faults, breakpoints, etc) and cause ptrace events via other
> paths.

What are you thinking for a solution?

As for capping SECCOMP_RET_ERRNO to MAX_ERRNO, how about this (sorry
if gmail butchers the paste):

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 4ef9687ac115..c88148d20bd5 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -629,7 +629,9 @@ static u32 __seccomp_phase1_filter(int this_syscall, struct

        switch (action) {
        case SECCOMP_RET_ERRNO:
-               /* Set the low-order 16-bits as a errno. */
+               /* Set the low-order bits as a errno. */
+               if (data > MAX_ERRNO)
+                       data = MAX_ERRNO;
                syscall_set_return_value(current, task_pt_regs(current),
                                         -data, 0);
                goto skip;


-- 
Kees Cook
Chrome OS Security