Re: [PATCH v2] keys: Protect the read method check against type removal

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Oct 19, 2026 at 10:57:21PM +0800, Cen Zhang wrote:
> keyctl_read_key() checks key->type->read without holding key->sem.
> A reference keeps the key alive, but does not pin the module containing
> its type. A surviving RxRPC key can therefore be read while AF_RXRPC
> is being unloaded:
> 
>   KEYCTL_READ task                    Module unload and key GC
>   ----------------                   ------------------------
>   lookup_user_key()
>   permission / possession checks
>   load key->type
>                                      af_rxrpc_exit()
>                                        unregister_key_type()
>                                          GC: down_write(key->sem)
>                                          GC: retype to key_type_dead
>                                          GC: up_write(key->sem)
>                                        unregister_key_type() returns
>                                      free_module()
>   read saved type's read member
> 
> The final access can read freed module storage. The read invocation in
> __keyctl_read_key() is already protected by key->sem, but the earlier
> method check is outside that critical section.
> 
> Move the method check into __keyctl_read_key(), after key_validate() and
> under its existing read hold on key->sem. The check and invocation then
> use the same protected type for length queries and every read retry.
> The GC cannot retype the key until the semaphore is released, and type
> unregistration waits for that GC pass before the module can be freed.
> 
> Return -EOPNOTSUPP when validation succeeds and the type has no read
> method. A temporary buffer may now be allocated before this result;
> allocation and copying to userspace remain outside the semaphore.
> 
> Suggested-by: Jarkko Sakkinen <jarkko@kernel.org>
> Assisted-by: LLM
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>

I literally described the fix to the reported bug instead of reviewing a
bug fix. That means by practical means that you only reported a bug.
This cannot happen again and if it does, next time I'll just do the
right fix myself and commit to the Git because that matches the reality
the best.


Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

> 
> ---
> Changes in v2:
> - Move the method check into __keyctl_read_key() under its existing read
>   hold on key->sem and remove the outer check.
> - Perform key validation before checking and invoking the read method.
> 
> Link to v1: https://lore.kernel.org/r/pm-key-management-objects-candidate-0010-v5-c6b7ca2813c6913e500a@gmail.com
> 
>  security/keys/keyctl.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 5a5efa26ed0e..36ffba0ebba6 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -808,8 +808,12 @@ static long __keyctl_read_key(struct key *key, char *buffer, size_t buflen)
>  
>  	down_read(&key->sem);
>  	ret = key_validate(key);
> -	if (ret == 0)
> -		ret = key->type->read(key, buffer, buflen);
> +	if (ret == 0) {
> +		if (key->type->read)
> +			ret = key->type->read(key, buffer, buflen);
> +		else
> +			ret = -EOPNOTSUPP;
> +	}
>  	up_read(&key->sem);
>  	return ret;
>  }
> @@ -863,11 +867,6 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>  
>  	/* the key is probably readable - now try to read it */
>  can_read_key:
> -	if (!key->type->read) {
> -		ret = -EOPNOTSUPP;
> -		goto key_put_out;
> -	}
> -
>  	if (!buffer || !buflen) {
>  		/* Get the key length from the read method */
>  		ret = __keyctl_read_key(key, NULL, 0);
> 
> base-commit: 89f5f41d296889f3b896c3ca67fcdf7d42bbdf10
> -- 
> 2.43.0

Br, Jarkko