Re: [BUG] net/ppp: A use after free in ppp_unregister_channe

On Thu, Mar 11, 2021 at 08:34:44PM +0800, lyl2019@mail.ustc.edu.cn wrote:

File: drivers/net/ppp/ppp_generic.c

In ppp_unregister_channel, pch could be freed in ppp_unbridge_channels()
but after that pch is still in use. Inside the function ppp_unbridge_channels,
if "pchbb == pch" is true and then pch will be freed.

No. It's freed only if if the refcount drops to 0. And the caller of
ppp_unregister_channel() must hold its own refcount. So
ppp_unbridge_channels() is not going to drop the last refcount in this
case.

I checked the commit history and found that this problem is introduced from
4cf476ced45d7 ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls").

Next time, please also Cc. the author of the patch.

I have no idea about how to generate a suitable patch, sorry.

There's no patch to send as far as I can see.