Re: use-after-free in sock_wake_async

[Eric Dumazet]

On Tue, Nov 24, 2015 at 5:10 PM, Rainer Weikusat
<rweikusat@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> The af_unix part of this, yes, ie, what gets allocated in
> unix_create1. But neither the socket inode nor the struct sock
> originally passed to unix_create. Since these are part of the same
> umbrella structure, they'll both be freed as consequence of the
> sock_release iput. As far as I can tell (I don't claim that I'm
> necessarily right on this, this is just the result of spending ca 2h
> reading the code with the problem report in mind and looking for
> something which could cause it), doing a sock_hold on the unix peer of
> the socket in unix_stream_sendmsg is indeed not needed, however, there's
> no additional reference to the inode or the struct sock accompanying it,
> ie, both of these will be freed by unix_release_sock. This also affects
> unix_dgram_sendmsg.
>
> It's also easy to verify: Swap the unix_state_lock and
> other->sk_data_ready and see if the issue still occurs. Right now (this
> may change after I had some sleep as it's pretty late for me), I don't
> think there's another local fix: The ->sk_data_ready accesses a
> pointer after the lock taken by the code which will clear and
> then later free it was released.

It seems that :

int sock_wake_async(struct socket *sock, int how, int band)

should really be changed to

int sock_wake_async(struct socket_wq *wq, int how, int band)

So that RCU rules (already present) apply safely.

sk->sk_socket is inherently racy (that is : racy without using
sk_callback_lock rwlock )

Other possibility would be _not_ calling sock_orphan() from unix_release_sock()
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