Re: [PATCH]: sctp: Fix skb_over_panic resulting from multiple invalid

[Vlad Yasevich <vladislav.yasevich@hp.com>]

Looks good.

Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>

-vlad

Neil Horman wrote:
> Ok, version 4
> 
> Change Notes:
> 1) Minor cleanups, from Vlads notes
> 
> Summary:
> 
> 
> 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, threadinfoÀ40f000 taskÀ370be0)
> <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 10-1173
> 
> I've tested the below fix and confirmed that it fixes the issue.  We move to a
> strategy whereby we allocate a fixed size error chunk and ignore errors we don't
> have space to report.  Tested by me successfully
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> 
> 
>  include/net/sctp/structs.h |    1 
>  net/sctp/sm_make_chunk.c   |   62 +++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 58 insertions(+), 5 deletions(-)
> 
> 
> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
> index ff30177..597f8e2 100644
> --- a/include/net/sctp/structs.h
> +++ b/include/net/sctp/structs.h
> @@ -778,6 +778,7 @@ int sctp_user_addto_chunk(struct sctp_chunk *chunk, int off, int len,
>  			  struct iovec *data);
>  void sctp_chunk_free(struct sctp_chunk *);
>  void  *sctp_addto_chunk(struct sctp_chunk *, int len, const void *data);
> +void  *sctp_addto_chunk_fixed(struct sctp_chunk *, int len, const void *data);
>  struct sctp_chunk *sctp_chunkify(struct sk_buff *,
>  				 const struct sctp_association *,
>  				 struct sock *);
> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> index f592163..2971731 100644
> --- a/net/sctp/sm_make_chunk.c
> +++ b/net/sctp/sm_make_chunk.c
> @@ -107,7 +107,7 @@ static const struct sctp_paramhdr prsctp_param = {
>  	cpu_to_be16(sizeof(struct sctp_paramhdr)),
>  };
>  
> -/* A helper to initialize to initialize an op error inside a
> +/* A helper to initialize an op error inside a
>   * provided chunk, as most cause codes will be embedded inside an
>   * abort chunk.
>   */
> @@ -124,6 +124,29 @@ void  sctp_init_cause(struct sctp_chunk *chunk, __be16 cause_code,
>  	chunk->subh.err_hdr = sctp_addto_chunk(chunk, sizeof(sctp_errhdr_t), &err);
>  }
>  
> +/* A helper to initialize an op error inside a
> + * provided chunk, as most cause codes will be embedded inside an
> + * abort chunk.  Differs from sctp_init_cause in that it won't oops
> + * if there isn't enough space in the op error chunk
> + */
> +int sctp_init_cause_fixed(struct sctp_chunk *chunk, __be16 cause_code,
> +		      size_t paylen)
> +{
> +	sctp_errhdr_t err;
> +	__u16 len;
> +
> +	/* Cause code constants are now defined in network order.  */
> +	err.cause = cause_code;
> +	len = sizeof(sctp_errhdr_t) + paylen;
> +	err.length  = htons(len);
> +
> +	if (skb_tailroom(chunk->skb) >  len)
> +		return -ENOSPC;
> +	chunk->subh.err_hdr = sctp_addto_chunk_fixed(chunk,
> +						     sizeof(sctp_errhdr_t),
> +						     &err);
> +	return 0;
> +}
>  /* 3.3.2 Initiation (INIT) (1)
>   *
>   * This chunk is used to initiate a SCTP association between two
> @@ -1131,6 +1154,24 @@ nodata:
>  	return retval;
>  }
>  
> +/* Create an Operation Error chunk of a fixed size,
> + * specifically, max(asoc->pathmtu, SCTP_DEFAULT_MAXSEGMENT)
> + * This is a helper function to allocate an error chunk for
> + * for those invalid parameter codes in which we may not want
> + * to report all the errors, if the incomming chunk is large
> + */
> +static inline struct sctp_chunk *sctp_make_op_error_fixed(
> +	const struct sctp_association *asoc,
> +	const struct sctp_chunk *chunk)
> +{
> +	size_t size = asoc ? asoc->pathmtu : 0;
> +
> +	if (!size)
> +		size = SCTP_DEFAULT_MAXSEGMENT;
> +
> +	return sctp_make_op_error_space(asoc, chunk, size);
> +}
> +
>  /* Create an Operation Error chunk.  */
>  struct sctp_chunk *sctp_make_op_error(const struct sctp_association *asoc,
>  				 const struct sctp_chunk *chunk,
> @@ -1373,6 +1414,18 @@ void *sctp_addto_chunk(struct sctp_chunk *chunk, int len, const void *data)
>  	return target;
>  }
>  
> +/* Append bytes to the end of a chunk. Returns NULL if there isn't sufficient
> + * space in the chunk
> + */
> +void *sctp_addto_chunk_fixed(struct sctp_chunk *chunk,
> +			     int len, const void *data)
> +{
> +	if (skb_tailroom(chunk->skb) > len)
> +		return sctp_addto_chunk(chunk, len, data);
> +	else
> +		return NULL;
> +}
> +
>  /* Append bytes from user space to the end of a chunk.  Will panic if
>   * chunk is not big enough.
>   * Returns a kernel err value.
> @@ -1976,13 +2029,12 @@ static sctp_ierror_t sctp_process_unk_param(const struct sctp_association *asoc,
>  		 * returning multiple unknown parameters.
>  		 */
>  		if (NULL = *errp)
> -			*errp = sctp_make_op_error_space(asoc, chunk,
> -					ntohs(chunk->chunk_hdr->length));
> +			*errp = sctp_make_op_error_fixed(asoc, chunk);
>  
>  		if (*errp) {
> -			sctp_init_cause(*errp, SCTP_ERROR_UNKNOWN_PARAM,
> +			sctp_init_cause_fixed(*errp, SCTP_ERROR_UNKNOWN_PARAM,
>  					WORD_ROUND(ntohs(param.p->length)));
> -			sctp_addto_chunk(*errp,
> +			sctp_addto_chunk_fixed(*errp,
>  					WORD_ROUND(ntohs(param.p->length)),
>  					param.v);
>  		} else {
>