common.c

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/***********************************************************************
*
* common.c
*
* Implementation of user-space PPPoE redirector for Linux.
*
* Common functions used by PPPoE client and server
*
* Copyright (C) 2000 by Roaring Penguin Software Inc.
*
* This program may be distributed according to the terms of the GNU
* General Public License, version 2 or (at your option) any later version.
*
* LIC: GPL
*
***********************************************************************/

static char const RCSID[] =
"$Id: common.c,v 1.19 2004/10/21 16:21:10 dfs Exp $";
/* For vsnprintf prototype */
#define _ISOC99_SOURCE 1

/* For seteuid prototype */
#define _BSD_SOURCE 1

#include "pppoe.h"


#ifdef HAVE_SYSLOG_H
#include <syslog.h>
#endif

#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdarg.h>

#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif

#include <sys/types.h>
#include <pwd.h>

/* Are we running SUID or SGID? */
int IsSetID = 0;

static uid_t saved_uid = -2;
static uid_t saved_gid = -2;

/**********************************************************************
*%FUNCTION: parsePacket
*%ARGUMENTS:
* packet -- the PPPoE discovery packet to parse
* func -- function called for each tag in the packet
* extra -- an opaque data pointer supplied to parsing function
*%RETURNS:
* 0 if everything went well; -1 if there was an error
*%DESCRIPTION:
* Parses a PPPoE discovery packet, calling "func" for each tag in the packet.
* "func" is passed the additional argument "extra".
***********************************************************************/
int
parsePacket(PPPoEPacket *packet, ParseFunc *func, void *extra)
{
    UINT16_t len = ntohs(packet->length);
    unsigned char *curTag;
    UINT16_t tagType, tagLen;

    if (packet->ver != 1) {
	syslog(LOG_ERR, "Invalid PPPoE version (%d)", (int) packet->ver);
	return -1;
    }
    if (packet->type != 1) {
	syslog(LOG_ERR, "Invalid PPPoE type (%d)", (int) packet->type);
	return -1;
    }

    /* Do some sanity checks on packet */
    if (len > ETH_DATA_LEN - 6) { /* 6-byte overhead for PPPoE header */
	syslog(LOG_ERR, "Invalid PPPoE packet length (%u)", len);
	return -1;
    }

    /* Step through the tags */
    curTag = packet->payload;
    while(curTag - packet->payload < len) {
	/* Alignment is not guaranteed, so do this by hand... */
	tagType = (((UINT16_t) curTag[0]) << 8) +
	    (UINT16_t) curTag[1];
	tagLen = (((UINT16_t) curTag[2]) << 8) +
	    (UINT16_t) curTag[3];
	if (tagType == TAG_END_OF_LIST) {
	    return 0;
	}
	if ((curTag - packet->payload) + tagLen + TAG_HDR_SIZE > len) {
	    syslog(LOG_ERR, "Invalid PPPoE tag length (%u)", tagLen);
	    return -1;
	}
	func(tagType, tagLen, curTag+TAG_HDR_SIZE, extra);
	curTag = curTag + TAG_HDR_SIZE + tagLen;
    }
    return 0;
}

/**********************************************************************
*%FUNCTION: findTag
*%ARGUMENTS:
* packet -- the PPPoE discovery packet to parse
* type -- the type of the tag to look for
* tag -- will be filled in with tag contents
*%RETURNS:
* A pointer to the tag if one of the specified type is found; NULL
* otherwise.
*%DESCRIPTION:
* Looks for a specific tag type.
***********************************************************************/
unsigned char *
findTag(PPPoEPacket *packet, UINT16_t type, PPPoETag *tag)
{
    UINT16_t len = ntohs(packet->length);
    unsigned char *curTag;
    UINT16_t tagType, tagLen;

    if (packet->ver != 1) {
	syslog(LOG_ERR, "Invalid PPPoE version (%d)", (int) packet->ver);
	return NULL;
    }
    if (packet->type != 1) {
	syslog(LOG_ERR, "Invalid PPPoE type (%d)", (int) packet->type);
	return NULL;
    }

    /* Do some sanity checks on packet */
    if (len > ETH_DATA_LEN - 6) { /* 6-byte overhead for PPPoE header */
	syslog(LOG_ERR, "Invalid PPPoE packet length (%u)", len);
	return NULL;
    }

    /* Step through the tags */
    curTag = packet->payload;
    while(curTag - packet->payload < len) {
	/* Alignment is not guaranteed, so do this by hand... */
	tagType = (((UINT16_t) curTag[0]) << 8) +
	    (UINT16_t) curTag[1];
	tagLen = (((UINT16_t) curTag[2]) << 8) +
	    (UINT16_t) curTag[3];
	if (tagType == TAG_END_OF_LIST) {
	    return NULL;
	}
	if ((curTag - packet->payload) + tagLen + TAG_HDR_SIZE > len) {
	    syslog(LOG_ERR, "Invalid PPPoE tag length (%u)", tagLen);
	    return NULL;
	}
	if (tagType == type) {
	    memcpy(tag, curTag, tagLen + TAG_HDR_SIZE);
	    return curTag;
	}
	curTag = curTag + TAG_HDR_SIZE + tagLen;
    }
    return NULL;
}

/**********************************************************************
*%FUNCTION: switchToRealID
*%ARGUMENTS:
* None
*%RETURNS:
* Nothing
*%DESCRIPTION:
* Sets effective user-ID and group-ID to real ones.  Aborts on failure
***********************************************************************/
void
switchToRealID (void) {
    if (IsSetID) {
	if (saved_uid < 0) saved_uid = geteuid();
	if (saved_gid < 0) saved_gid = getegid();
	if (setegid(getgid()) < 0) {
	    printErr("setgid failed");
	    exit(EXIT_FAILURE);
	}
	if (seteuid(getuid()) < 0) {
	    printErr("seteuid failed");
	    exit(EXIT_FAILURE);
	}
    }
}

/**********************************************************************
*%FUNCTION: switchToEffectiveID
*%ARGUMENTS:
* None
*%RETURNS:
* Nothing
*%DESCRIPTION:
* Sets effective user-ID and group-ID back to saved gid/uid
***********************************************************************/
void
switchToEffectiveID (void) {
    if (IsSetID) {
	if (setegid(saved_gid) < 0) {
	    printErr("setgid failed");
	    exit(EXIT_FAILURE);
	}
	if (seteuid(saved_uid) < 0) {
	    printErr("seteuid failed");
	    exit(EXIT_FAILURE);
	}
    }
}

/**********************************************************************
*%FUNCTION: dropPrivs
*%ARGUMENTS:
* None
*%RETURNS:
* Nothing
*%DESCRIPTION:
* If effective ID is root, try to become "nobody".  If that fails and
* we're SUID, switch to real user-ID
***********************************************************************/
void
dropPrivs(void)
{
    struct passwd *pw = NULL;
    int ok = 0;
    if (geteuid() == 0) {
	pw = getpwnam("nobody");
	if (pw) {
	    if (setgid(pw->pw_gid) < 0) ok++;
	    if (setuid(pw->pw_uid) < 0) ok++;
	}
    }
    if (ok < 2 && IsSetID) {
	setegid(getgid());
	seteuid(getuid());
    }
}

/**********************************************************************
*%FUNCTION: printErr
*%ARGUMENTS:
* str -- error message
*%RETURNS:
* Nothing
*%DESCRIPTION:
* Prints a message to stderr and syslog.
***********************************************************************/
void
printErr(char const *str)
{
    fprintf(stderr, "pppoe: %s\n", str);
    syslog(LOG_ERR, "%s", str);
}


/**********************************************************************
*%FUNCTION: strDup
*%ARGUMENTS:
* str -- string to copy
*%RETURNS:
* A malloc'd copy of str.  Exits if malloc fails.
***********************************************************************/
char *
strDup(char const *str)
{
    char *copy = malloc(strlen(str)+1);
    if (!copy) {
	rp_fatal("strdup failed");
    }
    strcpy(copy, str);
    return copy;
}

/**********************************************************************
*%FUNCTION: computeTCPChecksum
*%ARGUMENTS:
* ipHdr -- pointer to IP header
* tcpHdr -- pointer to TCP header
*%RETURNS:
* The computed TCP checksum
***********************************************************************/
UINT16_t
computeTCPChecksum(unsigned char *ipHdr, unsigned char *tcpHdr)
{
    UINT32_t sum = 0;
    UINT16_t count = ipHdr[2] * 256 + ipHdr[3];
    UINT16_t tmp;

    unsigned char *addr = tcpHdr;
    unsigned char pseudoHeader[12];

    /* Count number of bytes in TCP header and data */
    count -= (ipHdr[0] & 0x0F) * 4;

    memcpy(pseudoHeader, ipHdr+12, 8);
    pseudoHeader[8] = 0;
    pseudoHeader[9] = ipHdr[9];
    pseudoHeader[10] = (count >> 8) & 0xFF;
    pseudoHeader[11] = (count & 0xFF);

    /* Checksum the pseudo-header */
    sum += * (UINT16_t *) pseudoHeader;
    sum += * ((UINT16_t *) (pseudoHeader+2));
    sum += * ((UINT16_t *) (pseudoHeader+4));
    sum += * ((UINT16_t *) (pseudoHeader+6));
    sum += * ((UINT16_t *) (pseudoHeader+8));
    sum += * ((UINT16_t *) (pseudoHeader+10));

    /* Checksum the TCP header and data */
    while (count > 1) {
	memcpy(&tmp, addr, sizeof(tmp));