linux-kernel - Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL

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Message-ID: <202009221241.4C36E4EB@keescook>
Date:   Tue, 22 Sep 2020 12:42:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>, luto@...nel.org,
        tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, willy@...radead.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org,
        kernel@...labora.com
Subject: Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

On Tue, Sep 08, 2020 at 12:59:49AM -0400, Gabriel Krisman Bertazi wrote:
> Christian Brauner <christian.brauner@...ntu.com> writes:
> 
> > On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:
> >> index afe01e232935..3511c98a7849 100644
> >> --- a/include/linux/sched.h
> >> +++ b/include/linux/sched.h
> >> @@ -959,7 +959,11 @@ struct task_struct {
> >>  	kuid_t				loginuid;
> >>  	unsigned int			sessionid;
> >>  #endif
> >> -	struct seccomp			seccomp;
> >> +
> >> +	struct {
> >> +		unsigned int			syscall_intercept;
> >> +		struct seccomp			seccomp;
> >> +	};
> >
> > If there's no specific reason to do this I'd not wrap this in an
> > anonymous struct. It doesn't really buy anything and there doesn't seem
> > to be  precedent in struct task_struct right now. Also, if this somehow
> > adds padding it seems you might end up increasing the size of struct
> > task_struct more than necessary by accident? (I might be wrong
> > though.)
> 
> Hi Christian,
> 
> Thanks for your review on this and on the other patches of this series.
> 
> I wrapped these to prevent struct layout randomization from separating
> the flags field from seccomp, as they are going to be used together and
> I was trying to reduce overhead to seccomp entry due to two cache misses
> when reading this structure.  Measuring it seccomp_benchmark didn't show
> any difference with the unwrapped version, so perhaps it was a bit of
> premature optimization?

That should not be a thing to think about here. Structure randomization
already has a mode to protect against cache line issues. I would leave
this as just a new member; no wrapping struct.

-- 
Kees Cook