keys: make keyring indexes independent of kernel addresses

KEYCTL_READ returns the serial numbers in a keyring in associative-array
traversal order.  The first word of the index is a public, deterministic
hash of the type pointer, domain pointer and description.  An unprivileged
user can therefore insert keys with known descriptions, predict their
order for each possible KASLR slide, and recover the kernel image address.
On an x86-64 build with 16 MiB alignment, 45 user keys suffice to
distinguish the candidate slides.

Remove the addresses from the whole index, not just its first hash word.
Assign each key type and domain a nonzero 64-bit ID and hash those IDs and
the description with a once-initialized SipHash key.  Use the full IDs in
the later index chunks as well.  Truncating or hashing the IDs there would
lose the guarantee that distinct type/domain/description tuples have a
differing index bit.

Allocate IDs lazily since private key types need not be registered.  Cache
them in the index key so tree walks and copies of an existing index do not
need to dereference a type descriptor that may have been unregistered.
Keep the 32-bit chunk layout and diff_objects() bit offsets consistent,
and retain the separate root slot for keyrings used by recursive search.
The key comparison, permissions and userspace data format are unchanged;
only the address-dependent in-memory ordering is replaced.

Cc: stable@vger.kernel.org
Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Assisted-by: Codex:gpt-6-astra
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20261013231251.56170-1-kylebot@openai.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

2 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/key-type.h b/include/linux/key-type.h
index bb97bd3e5af4..c4f329b6ae3f 100644
--- a/include/linux/key-type.h
+++ b/include/linux/key-type.h
@@ -161,6 +161,7 @@ struct key_type {
 				     const void *in, const void *in2);
 
 	/* internal fields */
+	atomic64_t		index_id;	/* Opaque keyring index ID */
 	struct list_head	link;		/* link in types list */
 	struct lock_class_key	lock_class;	/* key->sem lock class */
 } __randomize_layout;
diff --git a/include/linux/key.h b/include/linux/key.h
index 81b8f05c6898..e40b5267658f 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -107,6 +107,7 @@ struct keyring_name;
 
 struct key_tag {
 	struct rcu_head		rcu;
+	atomic64_t		index_id;	/* Opaque keyring index ID */
 	refcount_t		usage;
 	bool			removed;	/* T when subject removed */
 };
@@ -129,6 +130,8 @@ struct keyring_index_key {
 	struct key_type		*type;
 	struct key_tag		*domain_tag;	/* Domain of operation */
 	const char		*description;
+	u64			type_id;	/* Cached index IDs */
+	u64			domain_id;
 };
 
 union key_payload {
@@ -251,6 +254,8 @@ struct key {
 			struct key_type	*type;		/* type of key */
 			struct key_tag	*domain_tag;	/* Domain of operation */
 			char		*description;
+			u64		type_id;
+			u64		domain_id;
 		};
 	};