Re: [PATCH 2/4] arch/x86: implement the process_vm_exec syscall

[Jann Horn <jannh@google.com>]

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 flags,
> > >                 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 *target_mm)
> > > +{
> > > +       struct task_struct *tsk = 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 = tsk->active_mm;
> > > +       if (active_mm != target_mm) {
> > > +               mmgrab(target_mm);
> > > +               tsk->active_mm = target_mm;
> > > +       }
> > > +       tsk->mm = 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’t 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?