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 13:52:05 -0400

Vlad Yasevich wrote:
> 
> 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. :)
> 

Hi Neil

I am also not crazy about the pre-allocation scheme.  In the case where you have
say 100 parameters that are all 'skip' parameters, you'd end up pre-allocating a
huge buffer for absolutely nothing.

This is another point toward a fixed error chunk size and let parameter
processing allocate it when it reaches a parameter that needs an error.

-vlad
> -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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