Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

Kees Cook
2018-10-30T21:32:41Z

On Tue, Oct 30, 2018 at 10:21 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> On Tue, Oct 30, 2018 at 05:39:26PM +0100, Oleg Nesterov wrote:
>> On 10/30, Oleg Nesterov wrote:
>> >
>> > On 10/30, Tycho Andersen wrote:
>> > >
>> > > @@ -828,6 +823,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>> > > */
>> > > rmb();
>> > >
>> > > + if (!sd) {
>> > > + populate_seccomp_data(&sd_local);
>> > > + sd = &sd_local;
>> > > + }
>> > > +
>> >
>> > To me it would be more clean to remove the "if (!sd)" check, case(SECCOMP_RET_TRACE)
>> > in __seccomp_filter() can simply do populate_seccomp_data(&sd_local) unconditionally
>> > and pass &sd_local to __seccomp_filter().
>>
>> Ah, please ignore, emulate_vsyscall() does secure_computing(NULL).

Right.

>>
>> Btw. why __seccomp_filter() doesn't return a boolean?

Because it was wrapped by __secure_computing(). *shrug* The common
method in the kernel is to use int and 0=ok.

>> Or at least, why can't case(SECCOMP_RET_TRACE) simply do
>>
>> return __seccomp_filter(this_syscall, NULL, true);
>>
>> ?
>
> Yeah, at least the second one definitely makes sense. I can add that
> as a patch in the next version of this series unless Kees does it
> before.

I'd like to avoid changing the return value of __secure_computing() to
just avoid having to touch all the callers. And I'd prefer not to
change __seccomp_filter() to a bool, since I'd like the return values
to be consistent through the call chain.

I find the existing code more readable than a single-line return, just
because it's very explicit. I don't want to have to think any harder
when reading seccomp. ;)

-Kees

--
Kees Cook