[PATCH net 1/1] net: ipconfig: bound DHCP option construction

[Yuqi Xu <xuyuqiabc@gmail.com>]

ic_dhcp_init_options() appends the hostname (option 12), vendor-class
(option 60) and client-ID (option 61) options into the fixed 312-byte
bootp_pkt.exten[] buffer.  Only the client-ID branch checked the
remaining space; the hostname and vendor-class writes were unbounded.

A 64-byte hostname together with the maximum 252-byte dhcpclass=
identifier needs 18 + (2 + 64) + (2 + 252) = 338 of the 312 available
bytes even before the terminating END marker, so the vendor-class memcpy
runs past the end of exten[].  With CONFIG_FORTIFY_SOURCE this is
reported as a field-spanning write and, when the kernel is booted with
panic_on_warn=1, aborts boot with a panic.

Route the optional options through a common helper that makes sure the
option, its 2-byte header and the END marker all fit and drops an option
that would not.  Configurations with short options keep sending exactly
the same bytes as before.

Fixes: 130c0f47fdf9 ("ipconfig: send host-name in DHCP requests")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Signed-off-by: Yuqi Xu <xuyuqiabc@gmail.com>
Reviewed-by: Ren Wei <weir@nebusec.ai>
---
 net/ipv4/ipconfig.c | 45 ++++++++++++++++++++++++++-------------------
 1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index 155db067eaec..1b8585404a41 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -676,6 +676,24 @@ static const u8 ic_bootp_cookie[4] = { 99, 130, 83, 99 };
 
 #ifdef IPCONFIG_DHCP
 
+static bool __init
+ic_dhcp_add_option(u8 **options, const u8 *end, u8 type, const void *value,
+		   int len)
+{
+	u8 *e = *options;
+
+	/* leave room for the option header and the END marker */
+	if (len > U8_MAX || end - e < len + 3)
+		return false;
+
+	*e++ = type;
+	*e++ = len;
+	memcpy(e, value, len);
+	*options = e + len;
+
+	return true;
+}
+
 static void __init
 ic_dhcp_init_options(u8 *options, struct ic_device *d)
 {
@@ -691,6 +709,7 @@ ic_dhcp_init_options(u8 *options, struct ic_device *d)
 		42,	/* NTP servers */
 	};
 	u8 mt = (ic_servaddr == NONE) ? DHCPDISCOVER : DHCPREQUEST;
+	u8 *end = options + sizeof(((struct bootp_pkt *)0)->exten);
 	u8 *e = options;
 	int len;
 
@@ -721,31 +740,19 @@ ic_dhcp_init_options(u8 *options, struct ic_device *d)
 	e += sizeof(ic_req_params);
 
 	if (ic_host_name_set) {
-		*e++ = 12;	/* host-name */
 		len = strlen(utsname()->nodename);
-		*e++ = len;
-		memcpy(e, utsname()->nodename, len);
-		e += len;
+		ic_dhcp_add_option(&e, end, 12, utsname()->nodename, len);
 	}
 	if (*vendor_class_identifier) {
-		pr_info("DHCP: sending class identifier \"%s\"\n",
-			vendor_class_identifier);
-		*e++ = 60;	/* Class-identifier */
 		len = strlen(vendor_class_identifier);
-		*e++ = len;
-		memcpy(e, vendor_class_identifier, len);
-		e += len;
+		if (ic_dhcp_add_option(&e, end, 60, vendor_class_identifier, len))
+			pr_info("DHCP: sending class identifier \"%s\"\n",
+				vendor_class_identifier);
 	}
 	len = strlen(dhcp_client_identifier + 1);
-	/* the minimum length of identifier is 2, include 1 byte type,
-	 * and can not be larger than the length of options
-	 */
-	if (len >= 1 && len < 312 - (e - options) - 1) {
-		*e++ = 61;
-		*e++ = len + 1;
-		memcpy(e, dhcp_client_identifier, len + 1);
-		e += len + 1;
-	}
+	/* the minimum length of identifier is 2, include 1 byte type */
+	if (len >= 1)
+		ic_dhcp_add_option(&e, end, 61, dhcp_client_identifier, len + 1);
 
 	*e++ = 255;	/* End of the list */
 }
-- 
2.55.0