Messages in this thread | | | From | guominchen(陈国民) <> | Subject | 答复: [PATCH] Export mm update next owner f unction for vhost-net(Internet mail) | Date | Thu, 13 Dec 2018 09:13:34 +0000 |
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>> 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.
Thank your for taking a look and apologize for my distrub.
>Could you describe how you reproduce this issue? Sorry, this issue is hard for my to reproduce, But there is such a critical situation.
>I believe vhost process should exit before process C? Yes, the A, B will exit before C, because usually C will close the open fd and then exit. However, if C is abnormally exited, such as killed by some fatal signal, A may exit before C
The current issue flow is as follows: Process C Process A Process B qemu-system-x86_64: kernel:vhost_net kernel: vhost_net open /dev/vhost-net VHOST_SET_OWNER create kthread vhost-%d create kthread vhost-%d network init use_mm() use_mm() ... ... Abnormal exited ... do_exit exit_mm() update mm->owner to A exit_files() close_files() kthread_should_stop() unuse_mm() Stop Process A tsk->mm=NULL do_exit() can't update owner A exit completed vhost-%d rcv first package vhost-%d build rcv buffer for vq page fault access mm & mm->owner NOW,mm->owner still pointer A kernel NULL pointer at mem_cgroup_from_task() stop Process B
>> >> 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()).
No, I think this is not racy. When page fault happend Between unuse_mm() and mm_update_next_owner(), Although tsk->mm =NULL, But tsk has not exited, So mm->onwer = tsk can still be accessed.
Thanks and regards
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