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

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Date:   Tue, 30 Oct 2018 14:32:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Tue, Oct 30, 2018 at 10:21 AM, Tycho Andersen <tycho@...ho.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