Subject: Re: [PATCH]: sctp: Fix skb_over_panic resulting from multiple invalid parameter errors (CVE-2010-1173)

From: Vlad Yasevich <[email-masked]>
Date: Wed, 28 Apr 2010 10:37:18 -0400

Neil Horman wrote:
> On Wed, Apr 28, 2010 at 10:00:37AM -0400, Vlad Yasevich wrote:
>> I have this patch and a few others already queued.
>>
>> I was planning on sending these today for stable.
>>
>> Here is the full list of stable patches I have:
>>
>> sctp: Fix oops when sending queued ASCONF chunks
>> sctp: fix to calc the INIT/INIT-ACK chunk length correctly is set
>> sctp: per_cpu variables should be in bh_disabled section
>> sctp: fix potential reference of a freed pointer
>> sctp: avoid irq lock inversion while call sk->sk_data_ready()
>>
>> -vlad
>>
> Are you sure?  this oops looks _very_ simmilar to the INIT/INIT-ACK length
> calculation oops described above, but is in fact different, and requires this
> patch, from what I can see.  The right fix might be in the ASCONF chunk patch
> you list above, but I don't see that in your tree at the moment, so I can't be
> sure.

As I said, I totally goofed when reading the description and I apologize.
However, I do one comment regarding the patch.

If the bad packet is REALLY long (I mean close to 65K IP limit), then
we'll end up allocating a supper huge skb in this case and potentially exceed
the IP length limitation.  Section 11.4 of rfc 4960 allows us to omit some
errors and limit the size of the packet.

I would recommend limiting this to MTU worth of potentiall errors.  This is
on top of what the INIT-ACK is going to carry, so at most we'll sent 2 MTUs
worth.  That's still a potential by amplification attack, but it's somewhat
mitigated.

Of course now we have to handle the case of checking for space before adding
an error cause. :)

-vlad

> 
> Neil
> 
>> Neil Horman wrote:
>>> Hey-
>>> 	Recently, it was reported to me that the kernel could oops in the
>>> following way:
>>>
>>> <5> kernel BUG at net/core/skbuff.c:91!
>>> <5> invalid operand: 0000 [#1]
>>> <5> Modules linked in: sctp netconsole nls_utf8 autofs4 sunrpc iptable_filter
>>> ip_tables cpufreq_powersave parport_pc lp parport vmblock(U) vsock(U) vmci(U)
>>> vmxnet(U) vmmemctl(U) vmhgfs(U) acpiphp dm_mirror dm_mod button battery ac md5
>>> ipv6 uhci_hcd ehci_hcd snd_ens1371 snd_rawmidi snd_seq_device snd_pcm_oss
>>> snd_mixer_oss snd_pcm snd_timer snd_page_alloc snd_ac97_codec snd soundcore
>>> pcnet32 mii floppy ext3 jbd ata_piix libata mptscsih mptsas mptspi mptscsi
>>> mptbase sd_mod scsi_mod
>>> <5> CPU:    0
>>> <5> EIP:    0060:[<c02bff27>]    Not tainted VLI
>>> <5> EFLAGS: 00010216   (2.6.9-89.0.25.EL) 
>>> <5> EIP is at skb_over_panic+0x1f/0x2d
>>> <5> eax: 0000002c   ebx: c033f461   ecx: c0357d96   edx: c040fd44
>>> <5> esi: c033f461   edi: df653280   ebp: 00000000   esp: c040fd40
>>> <5> ds: 007b   es: 007b   ss: 0068
>>> <5> Process swapper (pid: 0, threadinfo=c040f000 task=c0370be0)
>>> <5> Stack: c0357d96 e0c29478 00000084 00000004 c033f461 df653280 d7883180
>>> e0c2947d 
>>> <5>        00000000 00000080 df653490 00000004 de4f1ac0 de4f1ac0 00000004
>>> df653490 
>>> <5>        00000001 e0c2877a 08000800 de4f1ac0 df653490 00000000 e0c29d2e
>>> 00000004 
>>> <5> Call Trace:
>>> <5>  [<e0c29478>] sctp_addto_chunk+0xb0/0x128 [sctp]
>>> <5>  [<e0c2947d>] sctp_addto_chunk+0xb5/0x128 [sctp]
>>> <5>  [<e0c2877a>] sctp_init_cause+0x3f/0x47 [sctp]
>>> <5>  [<e0c29d2e>] sctp_process_unk_param+0xac/0xb8 [sctp]
>>> <5>  [<e0c29e90>] sctp_verify_init+0xcc/0x134 [sctp]
>>> <5>  [<e0c20322>] sctp_sf_do_5_1B_init+0x83/0x28e [sctp]
>>> <5>  [<e0c25333>] sctp_do_sm+0x41/0x77 [sctp]
>>> <5>  [<c01555a4>] cache_grow+0x140/0x233
>>> <5>  [<e0c26ba1>] sctp_endpoint_bh_rcv+0xc5/0x108 [sctp]
>>> <5>  [<e0c2b863>] sctp_inq_push+0xe/0x10 [sctp]
>>> <5>  [<e0c34600>] sctp_rcv+0x454/0x509 [sctp]
>>> <5>  [<e084e017>] ipt_hook+0x17/0x1c [iptable_filter]
>>> <5>  [<c02d005e>] nf_iterate+0x40/0x81
>>> <5>  [<c02e0bb9>] ip_local_deliver_finish+0x0/0x151
>>> <5>  [<c02e0c7f>] ip_local_deliver_finish+0xc6/0x151
>>> <5>  [<c02d0362>] nf_hook_slow+0x83/0xb5
>>> <5>  [<c02e0bb2>] ip_local_deliver+0x1a2/0x1a9
>>> <5>  [<c02e0bb9>] ip_local_deliver_finish+0x0/0x151
>>> <5>  [<c02e103e>] ip_rcv+0x334/0x3b4
>>> <5>  [<c02c66fd>] netif_receive_skb+0x320/0x35b
>>> <5>  [<e0a0928b>] init_stall_timer+0x67/0x6a [uhci_hcd]
>>> <5>  [<c02c67a4>] process_backlog+0x6c/0xd9
>>> <5>  [<c02c690f>] net_rx_action+0xfe/0x1f8
>>> <5>  [<c012a7b1>] __do_softirq+0x35/0x79
>>> <5>  [<c0107efb>] handle_IRQ_event+0x0/0x4f
>>> <5>  [<c01094de>] do_softirq+0x46/0x4d
>>>
>>> Its an skb_over_panic BUG halt that results from processing an init chunk in
>>> which too many of its variable length parameters are in some way malformed.
>>>
>>> The problem is in sctp_process_unk_param:
>>> if (NULL == *errp)
>>> 	*errp = sctp_make_op_error_space(asoc, chunk,
>>> 					 ntohs(chunk->chunk_hdr->length));
>>>
>>> 	if (*errp) {
>>> 		sctp_init_cause(*errp, SCTP_ERROR_UNKNOWN_PARAM,
>>> 				 WORD_ROUND(ntohs(param.p->length)));
>>> 		sctp_addto_chunk(*errp,
>>> 			WORD_ROUND(ntohs(param.p->length)),
>>> 				  param.v);
>>>
>>> When we allocate an error chunk, we assume that the worst case scenario requires
>>> that we have chunk_hdr->length data allocated, which would be correct nominally,
>>> given that we call sctp_addto_chunk for the violating parameter.  Unfortunately,
>>> we also, in sctp_init_cause insert a sctp_errhdr_t structure into the error
>>> chunk, so the worst case situation in which all parameters are in violation
>>> requires chunk_hdr->length+(sizeof(sctp_errhdr_t)*param_count) bytes of data.
>>>
>>> The result of this error is that a deliberately malformed packet sent to a
>>> listening host can cause a remote DOS, described in CVE-2010-1173:
>>> http://cve.mitre.org/cgi-bin/cvename.cgi?name=2010-1173
>>>
>>> I've tested the below fix and confirmed that it fixes the issue.  It
>>> pre-allocates the error chunk in sctp_verify_init, where we are able to count
>>> the total number of variable length parameters, so we know how many error
>>> headers we might need.  Then we simply use that chunk, if we find an error, or
>>> discard/free it if all the parameters are valid.  Applies on top of the
>>> lksctp-dev tree
>>>
>>> Signed-off-by: Neil Horman <[email-masked]>
>>>
>>>
>>>  sm_make_chunk.c |   24 ++++++++++++++++++++++--
>>>  1 file changed, 22 insertions(+), 2 deletions(-)
>>>
>>>
>>> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
>>> index f592163..990457b 100644
>>> --- a/net/sctp/sm_make_chunk.c
>>> +++ b/net/sctp/sm_make_chunk.c
>>> @@ -2134,6 +2134,8 @@ int sctp_verify_init(const struct sctp_association *asoc,
>>>  	union sctp_params param;
>>>  	int has_cookie = 0;
>>>  	int result;
>>> +	unsigned int param_cnt;
>>> +	unsigned int len;
>>>  
>>>  	/* Verify stream values are non-zero. */
>>>  	if ((0 == peer_init->init_hdr.num_outbound_streams) ||
>>> @@ -2149,6 +2151,7 @@ int sctp_verify_init(const struct sctp_association *asoc,
>>>  
>>>  		if (SCTP_PARAM_STATE_COOKIE == param.p->type)
>>>  			has_cookie = 1;
>>> +		param_cnt++;
>>>  
>>>  	} /* for (loop through all parameters) */
>>>  
>>> @@ -2169,6 +2172,20 @@ int sctp_verify_init(const struct sctp_association *asoc,
>>>  		return sctp_process_missing_param(asoc, SCTP_PARAM_STATE_COOKIE,
>>>  						  chunk, errp);
>>>  
>>> +	if (!*errp) {
>>> +		/*
>>> +		 * Pre-allocate the error packet here
>>> +		 * we do this as we need to reserve space
>>> +		 * for the worst case scenario in which 
>>> +		 * every parameter is in error and needs 
>>> +		 * an errhdr attached to it
>>> +		 */
>>> +		len = ntohs(chunk->chunk_hdr->length);
>>> +		len += sizeof(sctp_errhdr_t)*param_cnt;
>>> +
>>> +		*errp = sctp_make_op_error_space(asoc, chunk, len);
>>> +	}
>>> +
>>>  	/* Verify all the variable length parameters */
>>>  	sctp_walk_params(param, peer_init, init_hdr.params) {
>>>  
>>> @@ -2176,9 +2193,11 @@ int sctp_verify_init(const struct sctp_association *asoc,
>>>  		switch (result) {
>>>  		    case SCTP_IERROR_ABORT:
>>>  		    case SCTP_IERROR_NOMEM:
>>> -				return 0;
>>>  		    case SCTP_IERROR_ERROR:
>>> -				return 1;
>>> +				len = ntohs((*errp)->chunk_hdr->length);
>>> +				if ((*errp) && (len == sizeof(sctp_chunkhdr_t)))
>>> +					sctp_chunk_free(*errp);
>>> +				return (result == SCTP_IERROR_ERROR) ? 1 : 0;
>>>  		    case SCTP_IERROR_NO_ERROR:
>>>  		    default:
>>>  				break;
>>> @@ -2186,6 +2205,7 @@ int sctp_verify_init(const struct sctp_association *asoc,
>>>  
>>>  	} /* for (loop through all parameters) */
>>>  
>>> +	sctp_chunk_free(*errp);
>>>  	return 1;
>>>  }
>>>  
>>> --
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>