linux-kernel - Re: [patch V3 01/13] entry: Provide generic syscall entry functionality

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Message-ID: <87v9ijollo.fsf@nanos.tec.linutronix.de>
Date:   Sun, 19 Jul 2020 12:17:07 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Keno Fischer <keno@...iacomputing.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [patch V3 01/13] entry: Provide generic syscall entry functionality

Andy Lutomirski <luto@...nel.org> writes:
> On Sat, Jul 18, 2020 at 7:16 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Andy Lutomirski <luto@...nel.org> writes:
>> > FWIW, TIF_USER_RETURN_NOTIFY is a bit of an odd duck: it's an
>> > entry/exit word *and* a context switch word.  The latter is because
>> > it's logically a per-cpu flag, not a per-task flag, and the context
>> > switch code moves it around so it's always set on the running task.
>>
>> Gah, I missed the context switch thing of that. That stuff is hideous.
>
> It's also delightful because anything that screws up that dance (such
> as failure to do the exit-to-usermode path exactly right) likely
> results in an insta-root-hole.  If we fail to run user return
> notifiers, we can run user code with incorrect syscall MSRs, etc.

Looking at it deeper, having that thing in the loop is a pointless
exercise. This really wants to be done _after_ the loop.

Thanks,

        tglx