[PATCH v4 3/3] KEYS: Verify the key identity in __key_unlink_begin

[Jarkko Sakkinen]

When two keys have identical type and description, linking the second key
replaces the first in a keyring. However, attempting to unlink the first
key (no longer in the keyring) unlinks the second key.

This happens because __key_unlink_begin() calls assoc_array_delete() with
&key->index_key, matching only on type and description rather than the
key's identity.

Example:

  keyctl new_session >/dev/null
  keyctl newring ring_a @s
  keyctl newring ring_b @s
  A=$(keyctl add user A ss %:ring_a)
  B=$(keyctl add user A qq %:ring_b)
  keyctl newring R @s
  keyctl link $A %:R
  keyctl link $B %:R
  keyctl unlink $A %:R
  keyctl show

Verify the key identity before unlinking, and return -ENOENT on mismatch,
instead of unlinking the key with the wrong identity.

Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Reviewed-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
 security/keys/keyring.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 2c54f7f84c55..5281da9d5bf7 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -1506,9 +1506,15 @@ static int __key_unlink_begin(struct key *keyring, struct key *key,
 			      struct assoc_array_edit **_edit)
 {
 	struct assoc_array_edit *edit;
+	const void *object;
 
 	BUG_ON(*_edit != NULL);
 
+	object = assoc_array_find(&keyring->keys, &keyring_assoc_array_ops,
+				  &key->index_key);
+	if (!object || keyring_ptr_to_key(object) != key)
+		return -ENOENT;
+
 	edit = assoc_array_delete(&keyring->keys, &keyring_assoc_array_ops,
 				  &key->index_key);
 	if (IS_ERR(edit))
-- 
2.47.3