Re: [PATCH] tun: Fix use-after-free in tun_net_xmit

From: Jason Wang
Date: Sun May 05 2019 - 05:10:31 EST

On 2019/4/30 äå12:38, Cong Wang wrote:
On Sun, Apr 28, 2019 at 7:23 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:

On 2019/4/29 äå1:59, Cong Wang wrote:
On Sun, Apr 28, 2019 at 12:51 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
tun_net_xmit() doesn't have the chance to
access the change because it holding the rcu_read_lock().

The problem is the following codes:




We need make sure the decrement of tun->numqueues be visible to readers
after synchronize_net(). And in tun_net_xmit():
It doesn't matter at all. Readers are okay to read it even they still use the
stale tun->numqueues, as long as the tfile is not freed readers can read
whatever they want...
This is only true if we set SOCK_RCU_FREE, isn't it?

Sure, this is how RCU is supposed to work.

The decrement of tun->numqueues is just how we unpublish the old
tfile, it is still valid for readers to read it _after_ unpublish, we only need
to worry about free, not about unpublish. This is the whole spirit of RCU.

The point is we don't convert tun->numqueues to RCU but use
Why tun->numqueues needs RCU? It is an integer, and reading a stale
value is _perfectly_ fine.

I meant we don't want e.g tun_net_xmit() to see the stale value after synchronize_net() in __tun_detach(), since it has various other steps with the assumption that no tfile dereference from data path. E.g one example is XDP rxq information un-registering which looks racy in the case of XDP_TX.

If you actually meant to say tun->tfiles[] itself, no, it is a fixed-size array,
it doesn't shrink or grow, so we don't need RCU for it. This is also why
a stale tun->numqueues is fine, as long as it never goes out-of-bound.

We do kind of shrinking or growing through tun->numqueues. That's why we check against it in various places. But, of course this is buggy.

You need to rethink about my SOCK_RCU_FREE patch.
The code is wrote before SOCK_RCU_FREE is introduced and assume no
de-reference from device after synchronize_net(). It doesn't harm to
figure out the root cause which may give us more confidence to the fix
(e.g like SOCK_RCU_FREE).
I believe SOCK_RCU_FREE is the fix for the root cause, not just a

I don't object to fix with SOCK_RCU_FREE, but then we should remove
the redundant synchronize_net(). But I still prefer to synchronize
everything explicitly like (completely untested):
I agree that synchronize_net() can be removed. However I don't
understand your untested patch at all, it looks like to fix a completely
different problem rather than this use-after-free.

As has been mentioned, the problem of current code is that we still leave pointers to freed tfile in tfiles[] array in __tun_detach() and the check with tun->numqueues seems racy. So the patch just NULL out the detached tfile pointers and make sure no it can not be dereferenced from tfile after synchronize_net() by dereferencing tfile instead of checking tun->numqueues .