linux-kernel - Re: consistent oops from request

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Date:	Tue, 31 Mar 2009 13:53:34 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-cifs-client@...ts.samba.org
Subject: Re: consistent oops from request_key in 2.6.29

On Tue, 31 Mar 2009 18:44:05 +0100
David Howells <dhowells@...hat.com> wrote:

> 
> How about the attached patch?
> 
> David
> ---
> From: David Howells <dhowells@...hat.com>
> Subject: [PATCH] KEYS: Handle there being no fallback destination keyring for request_key()
> 
> When request_key() is called, without there being any standard process
> keyrings on which to fall back if a destination keyring is not specified, an
> oops is liable to occur when construct_alloc_key() calls down_write() on
> dest_keyring's semaphore.
> 
> Due to function inlining this may be seen as an oops in down_write() as called
> from request_key_and_link().
> 
> This situation crops up during boot, where request_key() is called from within
> the kernel (such as in CIFS mounts) where nobody is actually logged in, and so
> PAM has not had a chance to create a session keyring and user keyrings to act
> as the fallback.
> 
> To fix this, make construct_alloc_key() not attempt to cache a key if there is
> no fallback key if no destination keyring is given specifically.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
> 
>  security/keys/request_key.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
> 
> 
> diff --git a/security/keys/request_key.c b/security/keys/request_key.c
> index 22a3158..03fe63e 100644
> --- a/security/keys/request_key.c
> +++ b/security/keys/request_key.c
> @@ -311,7 +311,8 @@ static int construct_alloc_key(struct key_type *type,
>  
>  	set_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
>  
> -	down_write(&dest_keyring->sem);
> +	if (dest_keyring)
> +		down_write(&dest_keyring->sem);
>  
>  	/* attach the key to the destination keyring under lock, but we do need
>  	 * to do another check just in case someone beat us to it whilst we
> @@ -322,10 +323,12 @@ static int construct_alloc_key(struct key_type *type,
>  	if (!IS_ERR(key_ref))
>  		goto key_already_present;
>  
> -	__key_link(dest_keyring, key);
> +	if (dest_keyring)
> +		__key_link(dest_keyring, key);
>  
>  	mutex_unlock(&key_construction_mutex);
> -	up_write(&dest_keyring->sem);
> +	if (dest_keyring)
> +		up_write(&dest_keyring->sem);
>  	mutex_unlock(&user->cons_lock);
>  	*_key = key;
>  	kleave(" = 0 [%d]", key_serial(key));

That'll fix it. I tested an identical patch this morning...

Tested-by: Jeff Layton <jlayton@...hat.com>
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