[PATCH] keys, dns: Fix missing size check of V1 server-list header

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Hi Linus, Edward,

Here's Linus's patch dressed up with a commit message.  I would marginally
prefer just to insert the missing size check, but I'm also fine with Linus's
approach for now until we have different content types or newer versions.

Note that I'm not sure whether I should require Linus's S-o-b since he made
modifications or whether I should use a Codeveloped-by line for him.

From: Edward Adam Davis <eadavis@xxxxxx>

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

The dns_resolver_preparse() function has a check on the size of the payload
for the basic header of the binary-style payload, but is missing a check
for the size of the V1 server-list payload header after determining that's
what we've been given.

Fix this by getting rid of the the pointer to the basic header and just
assuming that we have a V1 server-list payload and moving the V1 server
list pointer inside the if-statement.  Dealing with other types and
versions can be left for when such have been defined.

This can be tested by doing the following with KASAN enabled:

        echo -n -e '\x0\x0\x1\x2' | keyctl padd dns_resolver foo @p

and produces an oops like the following:

        BUG: KASAN: slab-out-of-bounds in dns_resolver_preparse+0xc9f/0xd60 net/dns_resolver/dns_key.c:127
        Read of size 1 at addr ffff888028894084 by task syz-executor265/5069
        Call Trace:
         __dump_stack lib/dump_stack.c:88 [inline]
         dump_stack_lvl+0xd9/0x1b0 lib/dump_stack.c:106
         print_address_description mm/kasan/report.c:377 [inline]
         print_report+0xc3/0x620 mm/kasan/report.c:488
         kasan_report+0xd9/0x110 mm/kasan/report.c:601
         dns_resolver_preparse+0xc9f/0xd60 net/dns_resolver/dns_key.c:127
         __key_create_or_update+0x453/0xdf0 security/keys/key.c:842
         key_create_or_update+0x42/0x50 security/keys/key.c:1007
         __do_sys_add_key+0x29c/0x450 security/keys/keyctl.c:134
         do_syscall_x64 arch/x86/entry/common.c:52 [inline]
         do_syscall_64+0x40/0x110 arch/x86/entry/common.c:83

This patch was originally by Edward Adam Davis, but was modified by Linus.

Fixes: b946001d3bb1 ("keys, dns: Allow key types (eg. DNS) to be reclaimed immediately on expiry")
Reported-and-tested-by: syzbot+94bbb75204a05da3d89f@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/0000000000009b39bc060c73e209@xxxxxxxxxx/
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Tested-by: David Howells <dhowells@xxxxxxxxxx>
cc: Edward Adam Davis <eadavis@xxxxxx>
cc: Simon Horman <horms@xxxxxxxxxx>
cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
cc: Jeffrey E Altman <jaltman@xxxxxxxxxxxx>
cc: Wang Lei <wang840925@xxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: Steve French <sfrench@xxxxxxxxxx>
cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
cc: linux-cifs@xxxxxxxxxxxxxxx
cc: linux-nfs@xxxxxxxxxxxxxxx
cc: ceph-devel@xxxxxxxxxxxxxxx
cc: keyrings@xxxxxxxxxxxxxxx
cc: netdev@xxxxxxxxxxxxxxx
 net/dns_resolver/dns_key.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

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] == 0) {
+		const struct dns_server_list_v1_header *v1;
 		/* It may be a server list. */
-		if (datalen <= sizeof(*bin))
+		if (datalen <= sizeof(*v1))
 			return -EINVAL;
-		bin = (const struct dns_payload_header *)data;
-		kenter("[%u,%u],%u", bin->content, bin->version, datalen);
-		if (bin->content != DNS_PAYLOAD_IS_SERVER_LIST) {
+		v1 = (const struct dns_server_list_v1_header *)data;
+		kenter("[%u,%u],%u", v1->hdr.content, v1->hdr.version, datalen);
+		if (v1->hdr.content != DNS_PAYLOAD_IS_SERVER_LIST) {
 				"dns_resolver: Unsupported content type (%u)\n",
-				bin->content);
+				v1->hdr.content);
 			return -EINVAL;
-		if (bin->version != 1) {
+		if (v1->hdr.version != 1) {
 				"dns_resolver: Unsupported server list version (%u)\n",
-				bin->version);
+				v1->hdr.version);
 			return -EINVAL;
-		v1 = (const struct dns_server_list_v1_header *)bin;
 		if ((v1->status != DNS_LOOKUP_GOOD &&
 		     v1->status != DNS_LOOKUP_GOOD_WITH_BAD)) {
 			if (prep->expiry == TIME64_MAX)

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