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From:   Jann Horn <jannh@google.com>
Date:   Mon, 28 Jun 2021 18:13:29 +0200
Message-ID: <CAG48ez3UrzPE8rkucTgCu8ggcTEjx_h3Gj2FES1qM-uv2KD8bQ@mail.gmail.com>
Subject: Re: [PATCH 2/4] arch/x86: implement the process_vm_exec syscall
To:     Andrei Vagin <avagin@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-um@lists.infradead.org, criu@openvz.org, avagin@google.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.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 <peterz@infradead.org>,
        Richard Weinberger <richard@nod.at>,
        Thomas Gleixner <tglx@linutronix.de>
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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@.)

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);
> +}