On 2018/12/13 下午12:47, gchen.guomin@xxxxxxxxx wrote:
From: guomin chen <gchen.guomin@xxxxxxxxx>
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@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: guomin chen <gchen.guomin@xxxxxxxxx>
---
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 */
/*
