[PATCH v2] asymmetric_keys: check asymmetric_key_ids() for NULL before dereference

[Weiming Shi <bestswngs@gmail.com>]

asymmetric_key_ids() returns key->payload.data[asym_key_ids], which can
be NULL for keys parsed by the PKCS#8 parser (pkcs8_parser.c explicitly
stores NULL in prep->payload.data[asym_key_ids]).

key_or_keyring_common() in restrict.c and find_asymmetric_key() in
asymmetric_type.c both dereference this return value without checking
for NULL. An unprivileged user can trigger a NULL pointer dereference
in key_or_keyring_common() by creating a PKCS#8 key, restricting a
keyring with key_or_keyring:<pkcs8_serial>, and adding an X.509 cert
to the restricted keyring. CONFIG_PKCS8_PRIVATE_KEY_PARSER=y is
required.

The following bash script can reproduce the issue:

  #!/bin/bash
  modprobe pkcs8_key_parser 2>/dev/null
  openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:1024 \
      -out /tmp/poc.pem 2>/dev/null
  openssl pkcs8 -topk8 -nocrypt -in /tmp/poc.pem \
      -outform DER -out /tmp/poc.p8
  openssl req -new -x509 -key /tmp/poc.pem -outform DER \
      -out /tmp/poc.der -days 365 -subj "/CN=Test" \
      -addext "subjectKeyIdentifier=hash" \
      -addext "authorityKeyIdentifier=keyid:always" 2>/dev/null
  PKCS8_ID=$(keyctl padd asymmetric pkcs8key @s < /tmp/poc.p8)
  KR=$(keyctl newring test_kr @s)
  keyctl restrict_keyring $KR asymmetric "key_or_keyring:$PKCS8_ID"
  keyctl padd asymmetric trigger $KR < /tmp/poc.der
  rm -f /tmp/poc.pem /tmp/poc.p8 /tmp/poc.der

 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000
 KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 RIP: 0010:key_or_keyring_common (crypto/asymmetric_keys/restrict.c:205 crypto/asymmetric_keys/restrict.c:279)
 Call Trace:
  <TASK>
  __key_create_or_update (security/keys/key.c:884)
  key_create_or_update (security/keys/key.c:1021)
  __do_sys_add_key (security/keys/keyctl.c:134)
  do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
  </TASK>
 Kernel panic - not syncing: Fatal exception

Add a NULL check in find_asymmetric_key(), mirroring the existing
pattern in asymmetric_match_key_ids() and asymmetric_key_describe().
In key_or_keyring_common(), skip the trusted key matching when it
has no key IDs and fall through to the check_dest path.

Fixes: 7d30198ee24f ("keys: X.509 public key issuer lookup without AKID")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
v2: add bash reproducer to commit message (Ignat)

 crypto/asymmetric_keys/asymmetric_type.c | 2 ++
 crypto/asymmetric_keys/restrict.c        | 9 +++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/crypto/asymmetric_keys/asymmetric_type.c b/crypto/asymmetric_keys/asymmetric_type.c
index 16a7ae16593c..22f04656d529 100644
--- a/crypto/asymmetric_keys/asymmetric_type.c
+++ b/crypto/asymmetric_keys/asymmetric_type.c
@@ -109,6 +109,8 @@ struct key *find_asymmetric_key(struct key *keyring,
 	if (id_0 && id_1) {
 		const struct asymmetric_key_ids *kids = asymmetric_key_ids(key);
 
+		if (!kids)
+			goto reject;
 		if (!kids->id[1]) {
 			pr_debug("First ID matches, but second is missing\n");
 			goto reject;
diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index 86292965f493..ccf1084f720e 100644
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c
@@ -243,10 +243,14 @@ static int key_or_keyring_common(struct key *dest_keyring,
 			if (IS_ERR(key))
 				key = NULL;
 		} else if (trusted->type == &key_type_asymmetric) {
+			const struct asymmetric_key_ids *kids;
 			const struct asymmetric_key_id **signer_ids;
 
-			signer_ids = (const struct asymmetric_key_id **)
-				asymmetric_key_ids(trusted)->id;
+			kids = asymmetric_key_ids(trusted);
+			if (!kids)
+				goto skip_trusted;
+
+			signer_ids = (const struct asymmetric_key_id **)kids->id;
 
 			/*
 			 * The auth_ids come from the candidate key (the
@@ -290,6 +294,7 @@ static int key_or_keyring_common(struct key *dest_keyring,
 		}
 	}
 
+skip_trusted:
 	if (check_dest && !key) {
 		/* See if the destination has a key that signed this one. */
 		key = find_asymmetric_key(dest_keyring, sig->auth_ids[0],
-- 
2.43.0