asymmetric_lookup_restriction() resolves the serial in a
key_or_keyring restriction with key_lookup().  This can return a negative
or uninstantiated key as well as a positive one, and the restriction then
retains that key.

When the key has the asymmetric type, key_or_keyring_common() treats it as
a usable trust key and dereferences asymmetric_key_ids(trusted).  Negative
asymmetric keys have no payload, so linking an asymmetric key to the
restricted keyring causes a NULL pointer dereference.

A process can retain a negative asymmetric key after a request_key() upcall
fails and use its serial to restrict a keyring it owns.  Linking a valid
X.509 key to that keyring then triggers the fault without special
privileges.  On systems configured to panic on oops, this is a local denial
of service.

KASAN reports:

  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:key_or_keyring_common+0x688/0x880
  Call Trace:
   key_link+0x1e1/0x300
   keyctl_keyring_link+0x6e/0xc0
   __do_sys_keyctl+0x299/0x530

Reject keys that have not been positively instantiated before installing
the restriction.  The explicit NULL trust key used by
key_or_keyring:0:chain remains unaffected.

Fixes: 7e3c4d22083f ("KEYS: Restrict asymmetric key linkage using a specific keychain")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 crypto/asymmetric_keys/asymmetric_type.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/crypto/asymmetric_keys/asymmetric_type.c b/crypto/asymmetric_keys/asymmetric_type.c
index 16a7ae16593c9e..ce78dbc8b8c1cc 100644
--- a/crypto/asymmetric_keys/asymmetric_type.c
+++ b/crypto/asymmetric_keys/asymmetric_type.c
@@ -553,6 +553,11 @@ static struct key_restriction *asymmetric_lookup_restriction(
 				ret = ERR_CAST(key);
 				goto out;
 			}
+			if (!key_is_positive(key)) {
+				key_put(key);
+				ret = ERR_PTR(-ENOKEY);
+				goto out;
+			}
 		}
 
 		ret = asymmetric_restriction_alloc(link_fn, key);

base-commit: 3d6d817622b0a9721e3cc404df3469171582be13
-- 
2.54.0