[v2,2/4] slirp: Add sanity check for str option length

When user provides a long domainname or hostname that doesn't fit in the
DHCP packet, we mustn't overflow the response packet buffer. Instead,
report errors, following the g_warning() in the slirp->vdnssearch
branch.

Also check the strlen against 256 when initializing slirp, which limit
is also from the protocol where one byte represents the string length.
This gives an early error before the warning which is harder to notice
or diagnose.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 net/slirp.c   |  9 +++++++++
 slirp/bootp.c | 32 ++++++++++++++++++++++----------
 2 files changed, 31 insertions(+), 10 deletions(-)

On 2018-09-06 07:43, Fam Zheng wrote:
> When user provides a long domainname or hostname that doesn't fit in the
> DHCP packet, we mustn't overflow the response packet buffer. Instead,
> report errors, following the g_warning() in the slirp->vdnssearch
> branch.
> 
> Also check the strlen against 256 when initializing slirp, which limit
> is also from the protocol where one byte represents the string length.
> This gives an early error before the warning which is harder to notice
> or diagnose.
> 
> Reported-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  net/slirp.c   |  9 +++++++++
>  slirp/bootp.c | 32 ++++++++++++++++++++++----------
>  2 files changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/net/slirp.c b/net/slirp.c
> index 1e14318b4d..fd21dc728c 100644
> --- a/net/slirp.c
> +++ b/net/slirp.c
> @@ -365,6 +365,15 @@ static int net_slirp_init(NetClientState *peer, const char *model,
>          return -1;
>      }
>  
> +    if (vdomainname && strlen(vdomainname) > 255) {
> +        error_setg(errp, "'domainname' parameter cannot exceed 255 bytes");
> +        return -1;
> +    }
> +
> +    if (vhostname && strlen(vhostname) > 255) {
> +        error_setg(errp, "'vhostname' parameter cannot exceed 255 bytes");
> +        return -1;
> +    }
>  
>      nc = qemu_new_net_client(&net_slirp_info, peer, model, name);
>  
> diff --git a/slirp/bootp.c b/slirp/bootp.c
> index 9e7b53ba94..1e8185f0ec 100644
> --- a/slirp/bootp.c
> +++ b/slirp/bootp.c
> @@ -159,6 +159,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
>      struct in_addr preq_addr;
>      int dhcp_msg_type, val;
>      uint8_t *q;
> +    uint8_t *end;
>      uint8_t client_ethaddr[ETH_ALEN];
>  
>      /* extract exact DHCP msg type */
> @@ -240,6 +241,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
>      rbp->bp_siaddr = saddr.sin_addr; /* Server IP address */
>  
>      q = rbp->bp_vend;
> +    end = (uint8_t *)&rbp[1];
>      memcpy(q, rfc1533_cookie, 4);
>      q += 4;
>  
> @@ -292,24 +294,33 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
>  
>          if (*slirp->client_hostname) {
>              val = strlen(slirp->client_hostname);
> -            *q++ = RFC1533_HOSTNAME;
> -            *q++ = val;
> -            memcpy(q, slirp->client_hostname, val);
> -            q += val;
> +            if (q + val + 2 >= end) {
> +                g_warning("DHCP packet size exceeded, "
> +                    "omitting host name option.");
> +            } else {
> +                *q++ = RFC1533_HOSTNAME;
> +                *q++ = val;
> +                memcpy(q, slirp->client_hostname, val);
> +                q += val;
> +            }
>          }
>  
>          if (slirp->vdomainname) {
>              val = strlen(slirp->vdomainname);
> -            *q++ = RFC1533_DOMAINNAME;
> -            *q++ = val;
> -            memcpy(q, slirp->vdomainname, val);
> -            q += val;
> +            if (q + val + 2 >= end) {
> +                g_warning("DHCP packet size exceeded, "
> +                    "omitting domain name option.");
> +            } else {
> +                *q++ = RFC1533_DOMAINNAME;
> +                *q++ = val;
> +                memcpy(q, slirp->vdomainname, val);
> +                q += val;
> +            }
>          }
>  
>          if (slirp->vdnssearch) {
> -            size_t spaceleft = sizeof(rbp->bp_vend) - (q - rbp->bp_vend);
>              val = slirp->vdnssearch_len;
> -            if (val + 1 > spaceleft) {
> +            if (q + val >= end) {
>                  g_warning("DHCP packet size exceeded, "
>                      "omitting domain-search option.");
>              } else {
> @@ -331,6 +342,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
>          memcpy(q, nak_msg, sizeof(nak_msg) - 1);
>          q += sizeof(nak_msg) - 1;
>      }
> +    assert(q < end);
>      *q = RFC1533_END;
>  
>      daddr.sin_addr.s_addr = 0xffffffffu;
> 

Reviewed-by: Thomas Huth <thuth@redhat.com>

Since this can also fix potential QEMU crashes, I think this patch
should also go into the stable branches (put on CC: now).