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From: David Howells <dhowells@redhat.com>
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References: <0000000000009b39bc060c73e209@google.com>
To: syzbot <syzbot+94bbb75204a05da3d89f@syzkaller.appspotmail.com>
Cc: dhowells@redhat.com, davem@davemloft.net, edumazet@google.com,
    jarkko@kernel.org, jmorris@namei.org, keyrings@vger.kernel.org,
    kuba@kernel.org, linux-kernel@vger.kernel.org,
    linux-security-module@vger.kernel.org, netdev@vger.kernel.org,
    pabeni@redhat.com, paul@paul-moore.com, serge@hallyn.com,
    syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [net?] KASAN: slab-out-of-bounds Read in dns_resolver_preparse
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Date: Sat, 23 Dec 2023 23:17:13 +0000
Message-ID: <2591733.1703373433@warthog.procyon.org.uk>

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.gi=
t

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 2a6d363763a2..f18ca02aa95a 100644
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -91,8 +91,6 @@ const struct cred *dns_resolver_cache;
 static int
 dns_resolver_preparse(struct key_preparsed_payload *prep)
 {
-	const struct dns_server_list_v1_header *v1;
-	const struct dns_payload_header *bin;
 	struct user_key_payload *upayload;
 	unsigned long derrno;
 	int ret;
@@ -103,27 +101,28 @@ dns_resolver_preparse(struct key_preparsed_payload *=
prep)
 		return -EINVAL;
 =

 	if (data[0] =3D=3D 0) {
+		const struct dns_server_list_v1_header *v1;
+
 		/* It may be a server list. */
-		if (datalen <=3D sizeof(*bin))
+		if (datalen <=3D sizeof(*v1))
 			return -EINVAL;
 =

-		bin =3D (const struct dns_payload_header *)data;
-		kenter("[%u,%u],%u", bin->content, bin->version, datalen);
-		if (bin->content !=3D DNS_PAYLOAD_IS_SERVER_LIST) {
+		v1 =3D (const struct dns_server_list_v1_header *)data;
+		kenter("[%u,%u],%u", v1->hdr.content, v1->hdr.version, datalen);
+		if (v1->hdr.content !=3D DNS_PAYLOAD_IS_SERVER_LIST) {
 			pr_warn_ratelimited(
 				"dns_resolver: Unsupported content type (%u)\n",
-				bin->content);
+				v1->hdr.content);
 			return -EINVAL;
 		}
 =

-		if (bin->version !=3D 1) {
+		if (v1->hdr.version !=3D 1) {
 			pr_warn_ratelimited(
 				"dns_resolver: Unsupported server list version (%u)\n",
-				bin->version);
+				v1->hdr.version);
 			return -EINVAL;
 		}
 =

-		v1 =3D (const struct dns_server_list_v1_header *)bin;
 		if ((v1->status !=3D DNS_LOOKUP_GOOD &&
 		     v1->status !=3D DNS_LOOKUP_GOOD_WITH_BAD)) {
 			if (prep->expiry =3D=3D TIME64_MAX)