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SubjectRe: [PATCH] Export mm_update_next_owner function for vhost-net
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On 2018/12/13 下午12:47, gchen.guomin@gmail.com wrote:
> From: guomin chen <gchen.guomin@gmail.com>
>
> Under normal circumstances,When do_exit exits, mm->owner will
> be updated on exit_mm(). but when the kernel process calls
> unuse_mm() and then exits,mm->owner cannot be updated. And it
> will point to a task that has been released.
>
> Below is my issue on vhost_net:
> A, B are two kernel processes(such as vhost_worker),
> C is a user space process(such as qemu), and all
> three use the mm of the user process C.
> Now, because user process C exits abnormally, the owner of this
> mm becomes A. When A calls unuse_mm and exits, this mm->ower
> still points to the A that has been released.
> When B accesses this mm->owner again, A has been released.


Could you describe how you reproduce this issue? I believe vhost process
should exit before process C?


>
> Process A Process B
> vhost_worker() vhost_worker()
> --------- ---------
> use_mm() use_mm()
> ...
> unuse_mm()
> tsk->mm=NULL
> do_exit() page fault
> exit_mm() access mm->owner
> can't update owner kernel Oops
>
> unuse_mm()
>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: kvm@vger.kernel.org
> Cc: virtualization@lists.linux-foundation.org
> Cc: netdev@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
> Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: guomin chen <gchen.guomin@gmail.com>
> ---
> drivers/vhost/vhost.c | 1 +
> kernel/exit.c | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 6b98d8e..7c09087 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -368,6 +368,7 @@ static int vhost_worker(void *data)
> }
> }
> unuse_mm(dev->mm);
> + mm_update_next_owner(dev->mm);


If you analysis is correct, this is still racy isn't it? (E.g page fault
happen between unuse_mm() and mm_update_next_owner()).

Thanks


> set_fs(oldfs);
> return 0;
> }
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 0e21e6d..9e046dd 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -486,6 +486,7 @@ void mm_update_next_owner(struct mm_struct *mm)
> task_unlock(c);
> put_task_struct(c);
> }
> +EXPORT_SYMBOL(mm_update_next_owner);
> #endif /* CONFIG_MEMCG */
>
> /*

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