[PATCH] KEYS: Protect user key quota access during instantiation

[Cen Zhang]

The owner account must remain alive throughout payload quota
reservation. The user key type calls generic_key_instantiate(), which
dereferences key->user in key_payload_reserve(), without holding
key->sem. The construction mutex excludes other instantiators but does
not exclude keyctl_chown_key().

A kernel caller can allocate an in-quota user key, retain its reference
and instantiate it with a nonempty payload after publishing its serial.
A concurrent task with Setattr permission and CAP_SYS_ADMIN can chown
the still-uninstantiated key through KEY_LOOKUP_PARTIAL. If the key
holds the last reference to its non-root account, instantiation has no
authorization key, and the new owner has sufficient quota, this ordering
is possible:

  Instantiator                       Chown task
  key_instantiate_and_link()          keyctl_chown_key()
    lock key_construction_mutex        down_write(key->sem)
    generic_key_instantiate()
      key_payload_reserve()
        load old = key->user
                                       transfer quota to new owner
                                       replace key->user
                                       up_write(key->sem)
                                       key_put(key)
                                       key_user_put(old)
                                         kfree(old)
        read old->uid

Chown's final free falls between the old pointer load and the uid read.
The read can therefore access freed struct key_user storage despite the
caller retaining the key itself. Hold the read side of key->sem in a
local user instantiation callback across generic_key_instantiate().
Chown then cannot replace and release the account until the quota
accesses finish. If chown goes first, instantiation takes the semaphore
and uses the new account instead.

Keep locking out of the shared helper, which is also called by
big_key_update() with key->sem write-locked.

KASAN report as below:

    BUG: KASAN: slab-use-after-free in key_payload_reserve+0x54d/0x560


Fixes: 5801649d8b83 ("[PATCH] keys: let keyctl_chown() change a key's owner")
Signed-off-by: Cen Zhang <zzzccc427@xxxxxxxxx>
Assisted-by: LLM

---

diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 6f88b507f92791bd9569673c5d6534d76c58a978..b6e09921c11658c09eb7d7954b2abd5b2b7017c1 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -15,6 +15,7 @@
 #include "internal.h"
 
 static int logon_vet_description(const char *desc);
+static int user_instantiate(struct key *key, struct key_preparsed_payload *prep);
 
 /*
  * user defined keys take an arbitrary string as the description and an
@@ -24,7 +25,7 @@ struct key_type key_type_user = {
 	.name			= "user",
 	.preparse		= user_preparse,
 	.free_preparse		= user_free_preparse,
-	.instantiate		= generic_key_instantiate,
+	.instantiate		= user_instantiate,
 	.update			= user_update,
 	.revoke			= user_revoke,
 	.destroy		= user_destroy,
@@ -86,6 +87,19 @@ void user_free_preparse(struct key_preparsed_payload *prep)
 }
 EXPORT_SYMBOL_GPL(user_free_preparse);
 
+/*
+ * Keep the owner account stable while reserving quota during instantiation.
+ */
+static int user_instantiate(struct key *key, struct key_preparsed_payload *prep)
+{
+	int ret;
+
+	down_read(&key->sem);
+	ret = generic_key_instantiate(key, prep);
+	up_read(&key->sem);
+	return ret;
+}
+
 static void user_free_payload_rcu(struct rcu_head *head)
 {
 	struct user_key_payload *payload;