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

[Andy Lutomirski]

On Mon, Jun 28, 2021, at 9:13 AM, Jann Horn wrote:
> On Wed, Apr 14, 2021 at 7:59 AM Andrei Vagin <avagin@xxxxxxxxx> 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.