keys, dns: Fix size check of V1 server-list header

Fix the size check added to dns_resolver_preparse() for the V1 server-list
header so that it doesn't give EINVAL if the size supplied is the same as
the size of the header struct (which should be valid).

This can be tested with:

        echo -n -e '\0\0\01\xff\0\0' | keyctl padd dns_resolver desc @p

which will give "add_key: Invalid argument" without this fix.

Fixes: 1997b3cb ("keys, dns: Fix missing size check of V1 server-list header")
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Link: https://lore.kernel.org/r/ZZ4fyY4r3rqgZL+4@xpf.sh.intel.com/Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/dns_resolver/dns_key.c

@@ -104,7 +104,7 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
 		const struct dns_server_list_v1_header *v1;
 
 		/* It may be a server list. */
-		if (datalen <= sizeof(*v1))
+		if (datalen < sizeof(*v1))
 			return -EINVAL;
 
 		v1 = (const struct dns_server_list_v1_header *)data;