Re: [PATCH] keys: Pin request_key_auth payload in instantiate paths

On Tue, May 26, 2026 at 10:48:38AM +0800, Shaomin Chen wrote:
> keyctl_instantiate_key_common() reads request_key_auth from the assumed
> auth key before copying an instantiation payload from userspace.  The copy
> can fault and sleep.  If the request completes and revokes the auth key in
> that window, the auth payload can be detached and freed before the
> instantiate path uses it again.
> 
> A request-key helper reproducer can trigger this race.  One helper child
> blocks in KEYCTL_INSTANTIATE_IOV while the original helper instantiates the
> requested key and returns.  KASAN then reports a use-after-free from the
> stale request_key_auth payload in keyctl_instantiate_key_common().
> 
> Give request_key_auth payloads a refcount.  Take a payload reference while
> authkey->sem stabilizes the payload and revocation state.  Hold that
> reference across the instantiate and reject paths.  Drop the auth key
> owning reference from revoke and destroy.
> 
> Reported-by: Shaomin Chen <[email protected]>
> Closes: https://lore.kernel.org/r/[email protected]
> Signed-off-by: Shaomin Chen <[email protected]>
> ---
>  include/keys/request_key_auth-type.h |  2 ++
>  security/keys/internal.h             |  2 ++
>  security/keys/keyctl.c               | 24 +++++++++++++++-----
>  security/keys/request_key_auth.c     | 33 ++++++++++++++++++++++++++--
>  4 files changed, 53 insertions(+), 8 deletions(-)
> 
> diff --git a/include/keys/request_key_auth-type.h b/include/keys/request_key_auth-type.h
> index 36b89a933310..01e42ee5f409 100644
> --- a/include/keys/request_key_auth-type.h
> +++ b/include/keys/request_key_auth-type.h
> @@ -9,12 +9,14 @@
>  #define _KEYS_REQUEST_KEY_AUTH_TYPE_H
>  
>  #include <linux/key.h>
> +#include <linux/refcount.h>
>  
>  /*
>   * Authorisation record for request_key().
>   */
>  struct request_key_auth {
>  	struct rcu_head		rcu;
> +	refcount_t		usage;
>  	struct key		*target_key;
>  	struct key		*dest_keyring;
>  	const struct cred	*cred;
> diff --git a/security/keys/internal.h b/security/keys/internal.h
> index 2cffa6dc8255..b7b622bc36a1 100644
> --- a/security/keys/internal.h
> +++ b/security/keys/internal.h
> @@ -208,6 +208,8 @@ extern struct key *request_key_auth_new(struct key *target,
>  					const void *callout_info,
>  					size_t callout_len,
>  					struct key *dest_keyring);
> +struct request_key_auth *request_key_auth_get(struct key *authkey);
> +void request_key_auth_put(struct request_key_auth *rka);
>  
>  extern struct key *key_get_instantiation_authkey(key_serial_t target_id);
>  
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index ef855d69c97a..d14ace88e529 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -1197,9 +1197,13 @@ static long keyctl_instantiate_key_common(key_serial_t id,
>  	if (!instkey)
>  		goto error;
>  
> -	rka = instkey->payload.data[0];
> -	if (rka->target_key->serial != id)
> +	rka = request_key_auth_get(instkey);
> +	if (!rka) {
> +		ret = -EKEYREVOKED;
>  		goto error;
> +	}
> +	if (rka->target_key->serial != id)
> +		goto error_put_rka;
>  
>  	/* pull the payload in if one was supplied */
>  	payload = NULL;
> @@ -1208,7 +1212,7 @@ static long keyctl_instantiate_key_common(key_serial_t id,
>  		ret = -ENOMEM;
>  		payload = kvmalloc(plen, GFP_KERNEL);
>  		if (!payload)
> -			goto error;
> +			goto error_put_rka;
>  
>  		ret = -EFAULT;
>  		if (!copy_from_iter_full(payload, plen, from))
> @@ -1234,6 +1238,8 @@ static long keyctl_instantiate_key_common(key_serial_t id,
>  
>  error2:
>  	kvfree_sensitive(payload, plen);
> +error_put_rka:
> +	request_key_auth_put(rka);
>  error:
>  	return ret;
>  }
> @@ -1358,15 +1364,19 @@ long keyctl_reject_key(key_serial_t id, unsigned timeout, unsigned error,
>  	if (!instkey)
>  		goto error;
>  
> -	rka = instkey->payload.data[0];
> -	if (rka->target_key->serial != id)
> +	rka = request_key_auth_get(instkey);
> +	if (!rka) {
> +		ret = -EKEYREVOKED;
>  		goto error;
> +	}
> +	if (rka->target_key->serial != id)
> +		goto error_put_rka;
>  
>  	/* find the destination keyring if present (which must also be
>  	 * writable) */
>  	ret = get_instantiation_keyring(ringid, rka, &dest_keyring);
>  	if (ret < 0)
> -		goto error;
> +		goto error_put_rka;
>  
>  	/* instantiate the key and link it into a keyring */
>  	ret = key_reject_and_link(rka->target_key, timeout, error,
> @@ -1379,6 +1389,8 @@ long keyctl_reject_key(key_serial_t id, unsigned timeout, unsigned error,
>  	if (ret == 0)
>  		keyctl_change_reqkey_auth(NULL);
>  
> +error_put_rka:
> +	request_key_auth_put(rka);
>  error:
>  	return ret;
>  }
> diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
> index a7d7538c1f70..282e09d8fa46 100644
> --- a/security/keys/request_key_auth.c
> +++ b/security/keys/request_key_auth.c
> @@ -23,6 +23,7 @@ static void request_key_auth_describe(const struct key *, struct seq_file *);
>  static void request_key_auth_revoke(struct key *);
>  static void request_key_auth_destroy(struct key *);
>  static long request_key_auth_read(const struct key *, char *, size_t);
> +static void request_key_auth_rcu_disposal(struct rcu_head *);
>  
>  /*
>   * The request-key authorisation key type definition.
> @@ -115,6 +116,31 @@ static void free_request_key_auth(struct request_key_auth *rka)
>  	kfree(rka);
>  }
>  
> +/*
> + * Take a reference to the request-key authorisation payload so callers can
> + * drop authkey->sem before doing operations that may sleep.
> + */
> +struct request_key_auth *request_key_auth_get(struct key *authkey)
> +{
> +	struct request_key_auth *rka;
> +
> +	down_read(&authkey->sem);
> +	rka = dereference_key_locked(authkey);
> +	if (rka && !test_bit(KEY_FLAG_REVOKED, &authkey->flags))
> +		refcount_inc(&rka->usage);
> +	else
> +		rka = NULL;
> +	up_read(&authkey->sem);
> +
> +	return rka;
> +}
> +
> +void request_key_auth_put(struct request_key_auth *rka)
> +{
> +	if (rka && refcount_dec_and_test(&rka->usage))
> +		call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
> +}
> +
>  /*
>   * Dispose of the request_key_auth record under RCU conditions
>   */
> @@ -136,8 +162,10 @@ static void request_key_auth_revoke(struct key *key)
>  	struct request_key_auth *rka = dereference_key_locked(key);
>  
>  	kenter("{%d}", key->serial);
> +	if (!rka)
> +		return;
>  	rcu_assign_keypointer(key, NULL);
> -	call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
> +	request_key_auth_put(rka);
>  }
>  
>  /*
> @@ -150,7 +178,7 @@ static void request_key_auth_destroy(struct key *key)
>  	kenter("{%d}", key->serial);
>  	if (rka) {
>  		rcu_assign_keypointer(key, NULL);
> -		call_rcu(&rka->rcu, request_key_auth_rcu_disposal);
> +		request_key_auth_put(rka);
>  	}
>  }
>  
> @@ -174,6 +202,7 @@ struct key *request_key_auth_new(struct key *target, const char *op,
>  	rka = kzalloc_obj(*rka);
>  	if (!rka)
>  		goto error;
> +	refcount_set(&rka->usage, 1);
>  	rka->callout_info = kmemdup(callout_info, callout_len, GFP_KERNEL);
>  	if (!rka->callout_info)
>  		goto error_free_rka;
> -- 
> 2.47.3

I get the approach and for it makes sense but I'd really like to hear
David's feedback on this before moving forward.

In the meanwhile, were you able to do a script or similar as a
reproducer?

BR, Jarkko