[PATCH] KEYS: reject descriptions exceeding U16_MAX in __key_create_or_update()

[Hui Peng <benquike@gmail.com>]

struct keyring_index_key stores the key description length in a 16-bit
field (u16 desc_len), and __key_link_begin() enforces
BUG_ON(index_key->desc_len == 0).

While add_key() bounds explicit userspace description strings to
KEY_MAX_DESC_SIZE (4096), when the userspace description is NULL or
empty __key_create_or_update() delegates description generation to
index_key.type->preparse(&prep) on the payload (up to 1 MiB). Key types
such as "asymmetric" (x509_key_preparse()) construct prep.description
from the certificate issuer, subject, and hex-encoded serial number,
which can exceed 65535 bytes.

__key_create_or_update() then assigns:

        index_key.desc_len = strlen(index_key.description);

without checking whether the length fits in u16. When
strlen(index_key.description) is 65536 (0x10000), index_key.desc_len
truncates to 0 and immediately triggers BUG_ON(index_key->desc_len == 0)
in __key_link_begin(). Lengths above 65536 truncate modulo 65536,
corrupting keyring hash and comparison lengths.

Reject empty descriptions or descriptions longer than U16_MAX with
-EINVAL in __key_create_or_update() and key_alloc().

Fixes: 16feef434017 ("KEYS: Consolidate the concept of an 'index key' for key access")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
diff --git a/security/keys/key.c b/security/keys/key.c
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -245,6 +245,8 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 	}
 
 	desclen = strlen(desc);
+	if (desclen > U16_MAX)
+		goto error;
 	quotalen = desclen + 1 + type->def_datalen;
 
 	/* get hold of the key tracking for this user */
@@ -820,6 +822,7 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 	const struct cred *cred = current_cred();
 	struct key *keyring, *key = NULL;
 	key_ref_t key_ref;
+	size_t desc_len;
 	int ret;
 	struct key_restriction *restrict_link = NULL;
 
@@ -865,7 +868,11 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 		if (!index_key.description)
 			goto error_free_prep;
 	}
-	index_key.desc_len = strlen(index_key.description);
+	desc_len = strlen(index_key.description);
+	key_ref = ERR_PTR(-EINVAL);
+	if (!desc_len || desc_len > U16_MAX)
+		goto error_free_prep;
+	index_key.desc_len = desc_len;
 	key_set_index_key(&index_key);
 
 	ret = __key_link_lock(keyring, &index_key);
-- 
2.43.0