Re: [PATCH net 1/2] net: bpfilter: restart bpfilter_umh when error occurred

On Sat, 15 Dec 2018 at 09:58, David Miller [off-list ref] wrote:

From: Taehee Yoo <ap420073@gmail.com>
Date: Wed, 12 Dec 2018 10:19:26 +0900

quoted

When bpfilter error occurred bpfilter_umh will be stopped via __stop_umh().
The bpfilter_umh() couldn't start again because there is no restart
routine.

The section of bpfilter_umh_{start/end} is no longer .init.rodata
because these area should be reused in the restart routine. hence
the section name is changed to .bpfilter_umh.

Test commands:
   $ iptables -vnL
   $ kill -9 <pid of bpfilter_umh>
   $ iptables -vnL
   [  480.045136] bpfilter: write fail -32

   $ iptables -vnL
   iptables v1.8.1 (legacy): can't initialize iptables table `filter': No child processes
   Perhaps iptables or your kernel needs to be upgraded.

Then, iptables command is always failed.

Fixes: d2ba09c17a06 ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

Thank you for this fix, but I am unsure if this is a complete solution.

Thank you for your review!

First of all, you can only kill the bpfilter_umh as root right?

Yes, only root could kill bpfilter_umh process.

It is a big problem to allow the userspace to kill the umh program
because that will result in the pipes being leaked.  Normally the
kernel takes down bpfilter_umh and calls fput() on the pipe file
descriptors via shutdown_umh().

In the kill -9 example above, that does not happen and that is why
we get the fd leak.

In this situation it will not reset info->pid to zero, and it also
will leave bpfilter_process_socktop non-NULL.

Therefore, it seems like the kernel has to perform these cleanup
actions if the bpfilter_umh process dies (from kill -9, or just
crashing).  And I think if you manage that properly it will fix this
bug too.

If bpfilter_umh process is killed, shutdown_umh() is executed via __stop_umh().
because, __kernel_write() or kernel_read() will be failed in
__bpfilter_process_sockopt() if bpfilter_umh process had killed
or crashed. then, __bpfilter_process_sockopt() makes error message and
calls __stop_umh().
So the cleanup code is called when next iptables command is executed.

   $modprobe bpfilter
   $kill < pid of bpfilter_umh >
   $iptables -vnL   <-- stop_umh() is called and iptables command
fails at this point.
[  512.543626] bpfilter: write fail -32
   $iptables -vnL   <-- re-start routine will be called and iptables
command will success.

By any chance,  should the cleanup action immediately be called
when the bpfilter_umh process is killed?
If you think that I didn't understand correctly what you said,
please let me know.