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bgpd: fix mishandled attribute length

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A crafted BGP UPDATE with a malformed path attribute length field causes
bgpd to dump up to 65535 bytes of application memory and send it as the
data field in a BGP NOTIFY message, which is truncated to 4075 bytes
after accounting for protocol headers. After reading a malformed length
field, a NOTIFY is generated that is supposed to contain the problematic
data, but the malformed length field is inadvertently used to compute
how much data we send.

CVE-2017-15865

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>
This commit is contained in:
Quentin Young 2017-10-23 16:43:32 -04:00
parent 11567ce5a0
commit dacffad461
No known key found for this signature in database
GPG Key ID: DAF48E0F57E0834F
1 changed files with 38 additions and 2 deletions
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40
bgpd/bgp_attr.c
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@ -2281,10 +2281,46 @@ bgp_attr_parse_ret_t bgp_attr_parse(struct peer *peer, struct attr *attr,
"%s: BGP type %d length %d is too large, attribute total length is %d. attr_endp is %p. endp is %p",
peer->host, type, length, size, attr_endp,
endp);
/*
* RFC 4271 6.3
* If any recognized attribute has an Attribute
* Length that conflicts with the expected length
* (based on the attribute type code), then the
* Error Subcode MUST be set to Attribute Length
* Error. The Data field MUST contain the erroneous
* attribute (type, length, and value).
* ----------
* We do not currently have a good way to determine the
* length of the attribute independent of the length
* received in the message. Instead we send the
* minimum between the amount of data we have and the
* amount specified by the attribute length field.
*
* Instead of directly passing in the packet buffer and
* offset we use the stream_get* functions to read into
* a stack buffer, since they perform bounds checking
* and we are working with untrusted data.
*/
unsigned char ndata[BGP_MAX_PACKET_SIZE];
memset(ndata, 0x00, sizeof(ndata));
size_t lfl =
CHECK_FLAG(flag, BGP_ATTR_FLAG_EXTLEN) ? 2 : 1;
/* Rewind to end of flag field */
stream_forward_getp(BGP_INPUT(peer), -(1 + lfl));
/* Type */
stream_get(&ndata[0], BGP_INPUT(peer), 1);
/* Length */
stream_get(&ndata[1], BGP_INPUT(peer), lfl);
/* Value */
size_t atl = attr_endp - startp;
size_t ndl = MIN(atl, STREAM_READABLE(BGP_INPUT(peer)));
stream_get(&ndata[lfl + 1], BGP_INPUT(peer), ndl);
bgp_notify_send_with_data(
peer, BGP_NOTIFY_UPDATE_ERR,
BGP_NOTIFY_UPDATE_ATTR_LENG_ERR, startp,
attr_endp - startp);
BGP_NOTIFY_UPDATE_ATTR_LENG_ERR, ndata,
ndl + lfl + 1);
return BGP_ATTR_PARSE_ERROR;
}