| Message ID | 1544676445-14897-1-git-send-email-gchen.guomin@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Export mm_update_next_owner function for vhost-net | expand |
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 */ > > /*
>> 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
This seems like an issue all the unuse_mm users (at least those outside of swapfile.c) have, so it should be solved in the core. Bonus points for moving the set_fs magic into use_mm()..
On Tue, Dec 18, 2018 at 09:39:16AM +0800, gchen chen wrote: > Yes, I think so. > and i think the point is that unuse_mm() can't directly set tsk->mm=NULL. So why can't unuse_mm call mm_update_next_owner?
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); 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 */ /*