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

On Wed, Apr 14, 2021 at 7:59 AM Andrei Vagin [off-list ref] 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@.)

As far as I understand, only kthreads are allowed to do this (as
implemented in kthread_use_mm()).

+       switch_mm_irqs_off(active_mm, target_mm, tsk);
+       local_irq_enable();
+       task_unlock(tsk);
+#ifdef finish_arch_post_lock_switch
+       finish_arch_post_lock_switch();
+#endif
+
+       if (active_mm != target_mm)
+               mmdrop(active_mm);
+}