KEYS: fix dereferencing NULL payload with nonzero length

commit 5649645d upstream.

sys_add_key() and the KEYCTL_UPDATE operation of sys_keyctl() allowed a
NULL payload with nonzero length to be passed to the key type's
->preparse(), ->instantiate(), and/or ->update() methods.  Various key
types including asymmetric, cifs.idmap, cifs.spnego, and pkcs7_test did
not handle this case, allowing an unprivileged user to trivially cause a
NULL pointer dereference (kernel oops) if one of these key types was
present.  Fix it by doing the copy_from_user() when 'plen' is nonzero
rather than when '_payload' is non-NULL, causing the syscall to fail
with EFAULT as expected when an invalid buffer is specified.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

security/keys/keyctl.c

@@ -97,7 +97,7 @@ SYSCALL_DEFINE5(add_key, const char __user *, _type,
 	payload = NULL;
 
 	vm = false;
-	if (_payload) {
+	if (plen) {
 		ret = -ENOMEM;
 		payload = kmalloc(plen, GFP_KERNEL | __GFP_NOWARN);
 		if (!payload) {
@@ -331,7 +331,7 @@ long keyctl_update_key(key_serial_t id,
 
 	/* pull the payload in if one was supplied */
 	payload = NULL;
-	if (_payload) {
+	if (plen) {
 		ret = -ENOMEM;
 		payload = kmalloc(plen, GFP_KERNEL);
 		if (!payload)