linux-kernel - Re: [PATCH] CRED: Fix double free in prepare_usermodehelper

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Date:	Wed, 21 Apr 2010 09:47:04 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] CRED: Fix double free in
	prepare_usermodehelper_creds() error handling

* David Howells (dhowells@...hat.com) wrote:
> Patch 570b8fb505896e007fd3bb07573ba6640e51851d:
> 
> 	Author: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> 	Date:   Tue Mar 30 00:04:00 2010 +0100
> 	Subject: CRED: Fix memory leak in error handling
> 
> attempts to fix a memory leak in the error handling by making the offending
> return statement into a jump down to the bottom of the function where a
> kfree(tgcred) is inserted.
> 
> This is, however, incorrect, as it does a kfree() after doing put_cred() if
> security_prepare_creds() fails.  That will result in a double free if 'error'
> is jumped to as put_cred() will also attempt to free the new tgcred record by
> virtue of it being pointed to by the new cred record.

OK, I missed the fact taht put_cred() performs the kfree. Thanks for the fix.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
> 
>  kernel/cred.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/cred.c b/kernel/cred.c
> index e1dbe9e..ce1a52b 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -398,6 +398,8 @@ struct cred *prepare_usermodehelper_creds(void)
>  
>  error:
>  	put_cred(new);
> +	return NULL;
> +
>  free_tgcred:
>  #ifdef CONFIG_KEYS
>  	kfree(tgcred);
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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