Re: [PATCH 6/7] cifs.upcall: use ip address passed by kernel to get server's hostname

Jeff,
I'm sorry for a late response.

I see two possible issues with resolving ip to hostname and using it
as service principal:
1. In case of spoofed revers lookup or local dns cache or
compromised dns server or router between client and dns
server, we could be connected to a fake server without even
noticing it.
If we leave code as it is now, i.e. use hostname
supplied by user at mount time, we will get service ticket for
server we intended to connect to. In this case we at least will
get error message in logs  "Unexpected SMB signature".
As I can  remember, the current cifs code only checks signature
but doesn't tears session with wrong signature on packets
(transport.c:789) and doesn't do mutual authentication of the peer.

2.  It will break working now code In environments where there
is no dns or revers lookup doesn't match with a supplied
server name. And user will not even suspect why cifs doesn't
work anymore.

PS:
Theoretically we could have uninitialized memory read of
hostbuf if there wasn't ipv6 or ipv4 parameter in the upcall
request.

On Fri, Aug 7, 2009 at 11:43 PM, Jeff Layton<[email protected]> wrote:
> Instead of using the hostname given by the upcall to get the server's
> principal, take the IP address given in the upcall and reverse resolve
> it to a hostname.
>
> Signed-off-by: Jeff Layton <[email protected]>
> ---
>  client/cifs.upcall.c |   68 +++++++++++++++++++++++++++++++++++++++++---------
>  1 files changed, 56 insertions(+), 12 deletions(-)
>
> diff --git a/client/cifs.upcall.c b/client/cifs.upcall.c
> index 38be876..1e58503 100644
> --- a/client/cifs.upcall.c
> +++ b/client/cifs.upcall.c
> @@ -150,15 +150,15 @@ handle_krb5_mech(const char *oid, const char *principal, DATA_BLOB *secblob,
>  #define DKD_HAVE_HOSTNAME      0x1
>  #define DKD_HAVE_VERSION       0x2
>  #define DKD_HAVE_SEC           0x4
> -#define DKD_HAVE_IPV4          0x8
> -#define DKD_HAVE_IPV6          0x10
> -#define DKD_HAVE_UID           0x20
> -#define DKD_HAVE_PID           0x40
> -#define DKD_MUSTHAVE_SET (DKD_HAVE_HOSTNAME|DKD_HAVE_VERSION|DKD_HAVE_SEC)
> +#define DKD_HAVE_IP            0x8
> +#define DKD_HAVE_UID           0x10
> +#define DKD_HAVE_PID           0x20
> +#define DKD_MUSTHAVE_SET (DKD_HAVE_IP|DKD_HAVE_VERSION|DKD_HAVE_SEC)
>
>  static struct decoded_args {
>        int             ver;
>        char            *hostname;
> +       char            *ip;
>        uid_t           uid;
>        pid_t           pid;
>        sectype_t       sec;
> @@ -167,6 +167,7 @@ static struct decoded_args {
>  static unsigned int
>  decode_key_description(const char *desc, struct decoded_args *arg)
>  {
> +       int len;
>        int retval = 0;
>        char *pos;
>        const char *tkn = desc;
> @@ -174,7 +175,6 @@ decode_key_description(const char *desc, struct decoded_args *arg)
>        do {
>                pos = index(tkn, ';');
>                if (strncmp(tkn, "host=", 5) == 0) {
> -                       int len;
>
>                        if (pos == NULL)
>                                len = strlen(tkn);
> @@ -186,10 +186,18 @@ decode_key_description(const char *desc, struct decoded_args *arg)
>                        arg->hostname = SMB_XMALLOC_ARRAY(char, len);
>                        strlcpy(arg->hostname, tkn + 5, len);
>                        retval |= DKD_HAVE_HOSTNAME;
> -               } else if (strncmp(tkn, "ipv4=", 5) == 0) {
> -                       /* BB: do we need it if we have hostname already? */
> -               } else if (strncmp(tkn, "ipv6=", 5) == 0) {
> -                       /* BB: do we need it if we have hostname already? */
> +               } else if (!strncmp(tkn, "ip4=", 4) ||
> +                          !strncmp(tkn, "ip6=", 4)) {
> +                       if (pos == NULL)
> +                               len = strlen(tkn);
> +                       else
> +                               len = pos - tkn;
> +
> +                       len -= 3;
> +                       SAFE_FREE(arg->ip);
> +                       arg->ip = SMB_XMALLOC_ARRAY(char, len);
> +                       strlcpy(arg->ip, tkn + 4, len);
> +                       retval |= DKD_HAVE_IP;
>                } else if (strncmp(tkn, "pid=", 4) == 0) {
>                        errno = 0;
>                        arg->pid = strtol(tkn + 4, NULL, 0);
> @@ -288,6 +296,35 @@ cifs_resolver(const key_serial_t key, const char *key_descr)
>        return 0;
>  }
>
> +static int
> +ip_to_fqdn(const char *ipaddr, char *host, size_t hostlen)
> +{
> +       int rc;
> +       struct addrinfo hints = { .ai_flags = AI_NUMERICHOST };
> +       struct addrinfo *res;
> +
> +       rc = getaddrinfo(ipaddr, NULL, &hints, &res);
> +       if (rc) {
> +               syslog(LOG_DEBUG, "%s: failed to resolve %s to ipaddr: %s",
> +                       __func__, ipaddr,
> +                       rc == EAI_SYSTEM ? strerror(errno) : gai_strerror(rc));
> +               return rc;
> +       }
> +
> +       rc = getnameinfo(res->ai_addr, res->ai_addrlen, host, hostlen,
> +                        NULL, 0, NI_NAMEREQD);
> +       freeaddrinfo(res);
> +       if (rc) {
> +               syslog(LOG_DEBUG, "%s: failed to resolve %s to fqdn: %s",
> +                       __func__, ipaddr,
> +                       rc == EAI_SYSTEM ? strerror(errno) : gai_strerror(rc));
> +               return rc;
> +       }
> +
> +       syslog(LOG_DEBUG, "%s: resolved %s to %s", __func__, ipaddr, host);
> +       return 0;
> +}
> +
>  static void
>  usage(void)
>  {
> @@ -306,6 +343,7 @@ int main(const int argc, char *const argv[])
>        long rc = 1;
>        int c;
>        char *buf, *princ, *ccname = NULL;
> +       char hostbuf[NI_MAXHOST];
>        struct decoded_args arg = { };
>        const char *oid;
>
> @@ -383,12 +421,18 @@ int main(const int argc, char *const argv[])
>                }
>        }
>
> +       if (have & DKD_HAVE_IP) {
> +               rc = ip_to_fqdn(arg.ip, hostbuf, sizeof(hostbuf));
> +               if (rc)
> +                       goto out;
> +       }
> +
>        // do mech specific authorization
>        switch (arg.sec) {
>        case MS_KRB5:
>        case KRB5:
>                /* for "cifs/" service name + terminating 0 */
> -               datalen = strlen(arg.hostname) + 5 + 1;
> +               datalen = strnlen(hostbuf, sizeof(hostbuf)) + 5 + 1;
>                princ = SMB_XMALLOC_ARRAY(char, datalen);
>                if (!princ) {
>                        rc = 1;
> @@ -405,7 +449,7 @@ int main(const int argc, char *const argv[])
>                 * getting a host/ principal if that doesn't work.
>                 */
>                strlcpy(princ, "cifs/", datalen);
> -               strlcpy(princ + 5, arg.hostname, datalen - 5);
> +               strlcpy(princ + 5, hostbuf, datalen - 5);
>                rc = handle_krb5_mech(oid, princ, &secblob, &sess_key, ccname);
>                if (rc) {
>                        memcpy(princ, "host/", 5);
> --
> 1.6.0.6
>
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>
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