Re: panic in skb_push via sctp

From: Daniel Borkmann
Date: Mon Dec 01 2014 - 13:08:47 EST

On 12/01/2014 07:02 PM, Robert ÅwiÄcki wrote:
Thanks for looking into it. I can try with your patch, but no
guarantees that the fuzzer will hit the same condition in some
reasonable time-frame. Will get back in some time with results.

Ok, thanks!

PS. If you think it's possible to create a repro (userland code) which
can trigger this, I can give it a try.

Did by accident trinity create tunnels? It looks that upper layer
protocols (except SCTP) all allocate and reserve MAX_HEADER to
accommodate enough head room in worst case for possible tunnels.

2014-12-01 18:36 GMT+01:00 Daniel Borkmann <dborkman@xxxxxxxxxx>:
On 12/01/2014 05:49 PM, Robert ÅwiÄcki wrote:

I don't have much more, cause my kernel is kASLRNized and gdb cannot
handle that, but pasting output from kdb. Maybe somebody will be able
to see something obvious.

<0>[93699.703244] skbuff: skb_under_panic: text:ffffffff83cff03e
len:104 put:56 head:ffff8803bd804ec0 data:ffff8803bd804ebc tail:0x64
end:0xc0 dev:<NULL>

Thanks for the report!

On a first view, it looks like we should be using MAX_HEADER instead
of LL_MAX_HEADER here, could you try with the following patch:

diff --git a/net/sctp/output.c b/net/sctp/output.c
index 42dffd4..fc5e45b 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -401,12 +401,12 @@ int sctp_packet_transmit(struct sctp_packet *packet)
sk = chunk->skb->sk;

/* Allocate the new skb. */
- nskb = alloc_skb(packet->size + LL_MAX_HEADER, GFP_ATOMIC);
+ nskb = alloc_skb(packet->size + MAX_HEADER, GFP_ATOMIC);
if (!nskb)
goto nomem;

/* Make sure the outbound skb has enough header room reserved. */
- skb_reserve(nskb, packet->overhead + LL_MAX_HEADER);
+ skb_reserve(nskb, packet->overhead + MAX_HEADER);

/* Set the owning socket so that we know where to get the
* destination IP address.
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