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From:   Jann Horn <jannh@google.com>
Date:   Mon, 28 Jun 2021 19:14:31 +0200
Message-ID: <CAG48ez1d3abHEekh_fYoXdAe85RG+c--G7sEyMYhiKEsns3uog@mail.gmail.com>
Subject: Re: [PATCH 2/4] arch/x86: implement the process_vm_exec syscall
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Andrei Vagin <avagin@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-um@lists.infradead.org, criu@openvz.org, avagin@google.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Jeff Dike <jdike@addtoit.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Richard Weinberger <richard@nod.at>,
        Thomas Gleixner <tglx@linutronix.de>
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On Mon, Jun 28, 2021 at 6:30 PM Andy Lutomirski <luto@kernel.org> wrote:
> On Mon, Jun 28, 2021, at 9:13 AM, Jann Horn wrote:
> > On Wed, Apr 14, 2021 at 7:59 AM Andrei Vagin <avagin@gmail.com> wrote:
> > > This change introduces the new system call:
> > > process_vm_exec(pid_t pid, struct sigcontext *uctx, unsigned long fla=
gs,
> > >                 siginfo_t * uinfo, sigset_t *sigmask, size_t sizemask=
)
> > >
> > > process_vm_exec allows to execute the current process in an address
> > > space of another process.
> > [...]
> >
> > I still think that this whole API is fundamentally the wrong approach
> > because it tries to shoehorn multiple usecases with different
> > requirements into a single API. But that aside:
> >
> > > +static void swap_mm(struct mm_struct *prev_mm, struct mm_struct *tar=
get_mm)
> > > +{
> > > +       struct task_struct *tsk =3D current;
> > > +       struct mm_struct *active_mm;
> > > +
> > > +       task_lock(tsk);
> > > +       /* Hold off tlb flush IPIs while switching mm's */
> > > +       local_irq_disable();
> > > +
> > > +       sync_mm_rss(prev_mm);
> > > +
> > > +       vmacache_flush(tsk);
> > > +
> > > +       active_mm =3D tsk->active_mm;
> > > +       if (active_mm !=3D target_mm) {
> > > +               mmgrab(target_mm);
> > > +               tsk->active_mm =3D target_mm;
> > > +       }
> > > +       tsk->mm =3D target_mm;
> >
> > I'm pretty sure you're not currently allowed to overwrite the ->mm
> > pointer of a userspace thread. For example, zap_threads() assumes that
> > all threads running under a process have the same ->mm. (And if you're
> > fiddling with ->mm stuff, you should probably CC linux-mm@.)
>
> exec_mmap() does it, so it can=E2=80=99t be entirely impossible.

Yeah, true, execve can do it - I guess the thing that makes that
special is that it's running after de_thread(), so it's guaranteed to
be single-threaded?