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

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Date:   Wed, 23 Sep 2020 16:28:12 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Kees Cook <keescook@...omium.org>
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

Kees Cook <keescook@...omium.org> writes:

> 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.

Makes sense.  I will drop it for the next iteration.  Thanks!

-- 
Gabriel Krisman Bertazi