linux-kernel - Re: [PATCH] KEYS: put keyring if install_session_keyring_to

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Date:   Mon, 3 Apr 2017 10:48:57 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] KEYS: put keyring if install_session_keyring_to_cred()
 fails

Hi David,

On Mon, Apr 03, 2017 at 04:42:05PM +0100, David Howells wrote:
> Eric Biggers <ebiggers3@...il.com> wrote:
> 
> > +error3:
> > +	key_put(keyring);
> >  error2:
> >  	mutex_unlock(&key_session_mutex);
> 
> I would prefer the put to be outside of the locked section.  How about the
> attached instead?

[...]

> diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
> index 44451af828c0..8e98e5d1b5fb 100644
> --- a/security/keys/process_keys.c
> +++ b/security/keys/process_keys.c
> @@ -793,21 +793,20 @@ long join_session_keyring(const char *name)
>  			KEY_ALLOC_IN_QUOTA, NULL, NULL);
>  		if (IS_ERR(keyring)) {
>  			ret = PTR_ERR(keyring);
> -			goto error2;
> +			goto error_unlock_no_keyring;
>  		}
>  	} else if (IS_ERR(keyring)) {
>  		ret = PTR_ERR(keyring);
> -		goto error2;
> +		goto error_unlock_no_keyring;
>  	} else if (keyring == new->session_keyring) {
> -		key_put(keyring);
>  		ret = 0;
> -		goto error2;
> +		goto error_unlock;
>  	}
>  
>  	/* we've got a keyring - now to install it */
>  	ret = install_session_keyring_to_cred(new, keyring);
>  	if (ret < 0)
> -		goto error2;
> +		goto error_unlock;
>  
>  	commit_creds(new);
>  	mutex_unlock(&key_session_mutex);
> @@ -817,8 +816,11 @@ long join_session_keyring(const char *name)
>  okay:
>  	return ret;
>  
> -error2:
> +error_unlock_no_keyring:
> +	keyring = NULL;
> +error_unlock:
>  	mutex_unlock(&key_session_mutex);
> +	key_put(keyring);
>  error:
>  	abort_creds(new);
>  	return ret;

I guess I'm okay with either patch, but I think jumping to the error labels in
the way I proposed is more straightforward since it keeps things in the logical
order.  And I doubt there's much advantage in moving key_put() outside of the
mutex, given that key_session_mutex is only used for joining session keyrings so
shouldn't be contended, and key_put() hardly does any work anyway.

- Eric