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:This is only true if we set SOCK_RCU_FREE, isn't it?
It doesn't matter at all. Readers are okay to read it even they still use thetun_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():
stale tun->numqueues, as long as the tfile is not freed readers can read
whatever they want...
Sure, this is how RCU is supposed to work.
Why tun->numqueues needs RCU? It is an integer, and reading a staleThe decrement of tun->numqueues is just how we unpublish the oldThe point is we don't convert tun->numqueues to RCU but use
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.
value is _perfectly_ fine.
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.
I believe SOCK_RCU_FREE is the fix for the root cause, not just aYou 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 don't object to fix with SOCK_RCU_FREE, but then we should removeI agree that synchronize_net() can be removed. However I don't
the redundant synchronize_net(). But I still prefer to synchronize
everything explicitly like (completely untested):
understand your untested patch at all, it looks like to fix a completely
different problem rather than this use-after-free.