[PATCH 1/2] net: dns: fix packet buffer overflow from long hostnames

[Sascha Hauer <s.hauer@pengutronix.de>]

dns_send() uses strcpy() to copy the DNS-encoded hostname into the
packet buffer with no length check. The packet buffer has ~1482 bytes
available for the query name (PKTSIZE minus ethernet/IP/UDP headers,
DNS header, and query trailer). A hostname+domain combination longer
than this overflows the packet buffer.

The hostname comes from resolv() callers (TFTP server names, NFS
paths, user commands, etc.) and the domain from the user-configurable
global.net.domainname variable.

Fix by validating the encoded name length against the available packet
space before copying, and replace strcpy with memcpy using the known
length. Return -ENAMETOOLONG if the name doesn't fit.

Note the final component of the hostname is a dot ('.') which gets
replaced with a 0 in the "replace dots in fullname with chunk len" loop.
As we now calculate the strlen before that loop it will include that
final 0 already, so we must drop the "Mark end of host name" part.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 net/dns.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/net/dns.c b/net/dns.c
index 8fbd13cdc3..fde1439469 100644
--- a/net/dns.c
+++ b/net/dns.c
@@ -70,6 +70,10 @@ static int dns_send(const char *name)
 	unsigned char *packet = net_udp_get_payload(dns_con);
 	unsigned char *p, *s, *fullname, *dotptr;
 	const unsigned char *domain;
+	int namelen;
+	/* max UDP payload minus header and query trailer (5 bytes) */
+	const int maxlen = PKTSIZE - ETHER_HDR_SIZE - sizeof(struct iphdr) -
+			   sizeof(struct udphdr) - offsetof(struct header, data) - 5;
 
 	/* generate "difficult" to predict transaction id */
 	dns_req_id = dns_timer_start + (dns_timer_start >> 16);
@@ -90,6 +94,14 @@ static int dns_send(const char *name)
 	else
 		fullname = basprintf(".%s.", name);
 
+	namelen = strlen(fullname);
+	if (namelen > maxlen) {
+		pr_err("hostname too long for DNS query (%d > %d)\n",
+		       namelen, maxlen);
+		free(fullname);
+		return -ENAMETOOLONG;
+	}
+
 	/* replace dots in fullname with chunk len */
 	dotptr = fullname;
 	do {
@@ -104,11 +116,10 @@ static int dns_send(const char *name)
 	} while (*(dotptr + 1));
 	*dotptr = 0;
 
-	strcpy(header->data, fullname);
+	memcpy(header->data, fullname, namelen);
 
-	p = header->data + strlen(fullname);
+	p = header->data + namelen;
 
-	*p++ = 0;			/* Mark end of host name */
 	*p++ = 0;			/* Some servers require double null */
 	*p++ = (unsigned char)qtype;	/* Query Type */
 

-- 
2.47.3