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

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Date:   Sat, 18 Jul 2020 07:41:09 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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

On Sat, Jul 18, 2020 at 7:16 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Andy Lutomirski <luto@...nel.org> writes:
> > On Fri, Jul 17, 2020 at 12:29 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >> The alternative is to play nasty games with TIF_IA32, TIF_ADDR32 and
> >> TIF_X32 to free up bits for 32bit and make the flags field 64 bit on 64
> >> bit kernels, but I prefer to do the above seperation.
> >
> > I'm all for cleaning it up, but I don't think any nasty games would be
> > needed regardless.  IMO at least the following flags are nonsense and
> > don't belong in TIF_anything at all:
> >
> > TIF_IA32, TIF_X32: can probably be deleted.  Someone would just need
> > to finish the work.
> > TIF_ADDR32: also probably removable, but I'm less confident.
> > TIF_FORCED_TF: This is purely a ptrace artifact and could easily go
> > somewhere else entirely.
> >
> > So getting those five bits back would be straightforward.
> >
> > 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.