On 06/18/2013 01:45 PM, Neil Horman wrote:
On Tue, Jun 18, 2013 at 12:24:31PM -0400, Vlad Yasevich wrote:
On 06/18/2013 12:02 PM, Daniel Borkmann wrote:
On 06/18/2013 04:22 PM, Neil Horman wrote:
I like this idea, but I think I'm maybe missing something from it - we
reference
the socket in both the receive and send paths (sctp_unpack_cookie, is
specifically called from the rx path, which makes use of sp->hmac). a
socket
destructor can be called from __sk_free when sk_wmem_alloc reaches
zero, but we
use sk_refcnt in the rx path to prevent premature socket cleanup. If
we drain
our send queeue while wer'e still processing rx messages, what
prevents us from
freeing the socket in the tx path, via sk_free while we're still using
the
socket in the rx path. Note I don't think this patch is wrong per-se,
but it
seems to me there is more work to do to properly interlock the use of
sk_refcnt
and sk_wmem_alloc here (unless I'm just missing something obvious,
which is
entirely possible, I've been in the sun alot lately :) ).
Hm, __sk_free() calls sk_prot_free() which frees our socket structure
and in
sctp_wfree() we do a sctp_association_put(asoc) after sock_wfree(skb).
So no matter if having this patch or not, couldn't this use-after-free like
scenario already happen with the current code?
F.e. through a given call graph like that:
sctp_wfree(skb):
1) sock_wfree(skb)
-> __sk_free()
I don't think this can happen. sk_wmem_alloc is set to 1 in sk_alloc()
and that acts as a guard to make sure that sk_free() has been called
before we try to free things up. So, in this partcular case, for
__sk_free() to be called, sk_free() had to be called meaning the
last ref on the socket was released. However, that's not possible since
we are still holding the association and thus holding the socket
associated with it.
I see what your saying, and I agree, with that bias added in sk_alloc, it looks
like we won't ever call __sk_free until sk_wmem_alloc is 1 _and_ sk_refcnt is 0.
It still seems messy and confusing though. It would make more sense to me to
increment the refcount an additional time when the socket is initalized, and
then decrement it again when the socket is closed and sk_wmem_alloc reaches
zero. That would isolate the refcounting to a single variable.
See commit 2b85a34e911bf483c27cfdd124aeb1605145dc80. The whole
sk_wmem_alloc tric was done so that we dont have to do
sock_hold/sock_put on transmits.
It might be good to see if we can do that in sctp as well.
-vlad
Neil
-vlad
-> sk_prot_free(.., sk)
-> kmem_cache_free(.., sk) or kfree(sk)
2) __sctp_write_space(asoc)
3) sctp_association_put(asoc)
-> sctp_association_destroy(asoc)
-> sctp_endpoint_put(asoc->ep)
-> sctp_endpoint_destroy(ep)
-> crypto_free_hash(sctp_sk(ep->base.sk)->hmac)
(etc, all unconditionally accessed while sk is
already dead/freed)
Then, this might need a fix in general. :-)
Assuming you would reduce the buffer space via setsockopt(.., SO_SNDBUF,
..),
you might end up with a minimum buffer space of SOCK_MIN_SNDBUF [*] and
a call to
sk->sk_write_space(sk), which is sctp_write_space() and calls
__sctp_write_space()
on all asocs belonging to the socket, but it seems not to alter the current
sk->sk_wmem_alloc I think, but rather sk->sk_sndbuf.
[*] Btw, shouldn't this rather be (2048 + sizeof(struct sk_buff)) or
SKB_TRUESIZE(2048), at least like in SOCK_MIN_RCVBUF since we operate
on skb->truesize as well?
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