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>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

2 files changed, 31 insertions, 10 deletions

diff --git a/net/slirp.c b/net/slirp.c
index 99884de..da6c0a1 100644
--- a/net/slirp.c
+++ b/net/slirp.c
@@ -350,6 +350,15 @@ 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 9e7b53b..1e8185f 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;