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

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Message-ID: <202009251327.587D111@keescook>
Date:   Fri, 25 Sep 2020 13:30:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, luto@...nel.org,
        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 Fri, Sep 25, 2020 at 12:15:54PM -0400, Gabriel Krisman Bertazi wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> 
> > On Wed, Sep 23 2020 at 13:49, Kees Cook wrote:
> >> On Wed, Sep 23, 2020 at 04:18:26PM -0400, Gabriel Krisman Bertazi wrote:
> >>> Kees Cook <keescook@...omium.org> writes:
> >>> Yes, we can, and I'm happy to follow up with that as part of my TIF
> >>> clean up work, but can we not block the current patchset to be merged
> >>> waiting for that, as this already grew a lot from the original feature
> >>> submission?
> >>
> >> In that case, I'd say just add the new TIF flag. The consolidation can
> >> come later.
> >
> > No. This is exactly the wrong order. Cleanup and consolidation have
> > precedence over features. I'm tired of 'we'll do that later' songs,
> > simply because in the very end I'm going to be the idiot who mops up the
> > resulting mess.
> >
> 
> No problem.  I will follow up with a patchset consolidating those flags
> into this syscall_intercept interface I proposed.  I assume there is no
> immediate concerns with the consolidation approach itself.

I think the only issue is just finding a clean way to set/unset the
flags safely/quickly (a lock seems too heavy to me).

Should thread_info hold an entire u32 for all intercept flags (then the
TIF_WORK tests is just a zero-test of the intercept u32 word)? Or should
there be a TIF_INTERCEPT and a totally separate u32 (e.g. in
task_struct) indicating which intercepts? (And if they're separate, how
do we atomically set/unset)

i.e.:

atomic_start
	toggle a per-intercept bit
	set TIF_INTERCEPT = !!(intercept word)
atomic_end

-- 
Kees Cook