Re: [PATCH] bpf: whitelist syscalls for error injection

[Andy Lutomirski <luto@kernel.org>]

On Sun, Mar 18, 2018 at 6:47 AM, Dominik Brodowski
<linux@dominikbrodowski.net> wrote:
> On Fri, Mar 16, 2018 at 03:55:04PM -0700, Howard McLauchlan wrote:
>> On 03/13/2018 04:56 PM, Andy Lutomirski wrote:
>> > On Tue, Mar 13, 2018 at 11:16 PM, Howard McLauchlan <hmclauchlan@fb.com> wrote:
>> >> Error injection is a useful mechanism to fail arbitrary kernel
>> >> functions. However, it is often hard to guarantee an error propagates
>> >> appropriately to user space programs. By injecting into syscalls, we can
>> >> return arbitrary values to user space directly; this increases
>> >> flexibility and robustness in testing, allowing us to test user space
>> >> error paths effectively.
>> >
>> > Temporary NAK IMO.  Specifically:
>> >
>> >> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> >> index a78186d826d7..e8c6d63ace78 100644
>> >> --- a/include/linux/syscalls.h
>> >> +++ b/include/linux/syscalls.h
>> >> @@ -191,6 +191,8 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
>> >>
>> >>  #define SYSCALL_DEFINE0(sname)                                 \
>> >>         SYSCALL_METADATA(_##sname, 0);                          \
>> >> +       asmlinkage long sys_##sname(void);                      \
>> >> +       ALLOW_ERROR_INJECTION(sys_##sname, ERRNO);              \
>> >
>> > sys_xyz() is not just the syscall itself; it's also a helper that's
>> > used for entirely silly reasons by various bits of kernel code for
>> > quite a few syscalls.  Fortunately, Dominik has patches to fix that,
>> > and Linus is even considering pulling them for 4.16.  This patch will
>> > most likely conflict with the final result of Dominik's series.
>> >
>> > Can you and Dominik coordinate a bit to get this patch or its
>> > equivalent landed on top of Dominik's work?  It might make sense for
>> > Dominik to just add this patch to his series so it can land with the
>> > rest of it.  Dominik, Ingo, what do you think?
>> >
>> > --Andy
>> >
>>
>> Dominik,
>>
>> This patch applies cleanly on top of your patch series. Is there anything you'd need from me to get this in on top of your work?
>
> Howard,
>
> would this form part of the kernel<->userspace interface and therefore needs
> to be kept stable? If so, this patch should wait until the arch-specific
> syscall calling convention is agreed upon.
>
> Moreover, the patches I sent out already do not cover all syscalls yet.
> Until all in-kernel users of sys_*() are gone (or at least outside arch/),
> I'd prefer to postpone this patch.
>

I was assuming that this ALLOW_ERROR_INJECTION thing is *not*
considered stable ABI.  We should be free to change the way that the
syscall entry code calls syscalls whenever we like.

If you want a stable syscall error injection mechanism, make it work
like seccomp instead, please.