[PATCH] keys: reject descriptions that exceed the index length

struct keyring_index_key::desc_len is a u16.  User-provided key
descriptions are limited to 4095 bytes, but a key type preparser can
generate a longer description when the caller passes NULL.

The X.509 parser forms a description from the certificate subject and
twice the raw serial length.  A certificate with a two-byte subject and a
32766-byte serial therefore produces a 65536-byte description.  Assigning
strlen() to desc_len wraps it to zero, after which __key_link_begin() hits:

    BUG_ON(index_key->desc_len == 0);

This is reachable through add_key() by an unprivileged user and can panic
the kernel when oopses are fatal.

Measure generated descriptions before narrowing the length and reject
values that cannot be represented.  The boundary input now returns EINVAL,
while the one-byte-short control still reaches the normal quota check.

Fixes: f771fde82051 ("keys: Simplify key description management")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <redacted>
---
 security/keys/key.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/security/keys/key.c b/security/keys/key.c
index b34a64d81d47ab..f2f472b45f4eee 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c

@@ -14,6 +14,7 @@
 #include <linux/workqueue.h>
 #include <linux/random.h>
 #include <linux/err.h>
+#include <linux/limits.h>
 #include "internal.h"
 
 struct kmem_cache *key_jar;

@@ -820,6 +821,7 @@ 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 +867,12 @@ 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);
+	if (desc_len > U16_MAX) {
+		key_ref = ERR_PTR(-EINVAL);
+		goto error_free_prep;
+	}
+	index_key.desc_len = desc_len;
 	key_set_index_key(&index_key);
 
 	ret = __key_link_lock(keyring, &index_key);

base-commit: 0a0d1d55dad570724bf8c7ea83409639cfb4be9b