linux-kernel - Re: [PATCH] selinux: fix another double free

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Date:   Thu, 11 Jun 2020 19:12:47 +0200
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Tom Rix <trix@...hat.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Wei Yongjun <weiyongjun1@...wei.com>,
        SElinux list <selinux@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] selinux: fix another double free

Hi Tom,

On Thu, Jun 11, 2020 at 5:58 PM <trix@...hat.com> wrote:
> From: Tom Rix <trix@...hat.com>
>
> Clang static analysis reports this double free error
>
> security/selinux/ss/conditional.c:139:2: warning: Attempt to free released memory [unix.Malloc]
>         kfree(node->expr.nodes);
>         ^~~~~~~~~~~~~~~~~~~~~~~
>
> When cond_read_node fails, it calls cond_node_destroy which frees the
> node but does not poison the entry in the node list.  So when it
> returns to its caller cond_read_list, cond_read_list deletes the
> partial list.  The latest entry in the list will be deleted twice.
>
> So instead of freeing the node in cond_read_node, let list freeing in
> code_read_list handle the freeing the problem node along with all of the the

There is an extra "the" before "freeing".

> earlier nodes.
>
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  security/selinux/ss/conditional.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/security/selinux/ss/conditional.c b/security/selinux/ss/conditional.c
> index da94a1b4bfda..ffb31af22f4f 100644
> --- a/security/selinux/ss/conditional.c
> +++ b/security/selinux/ss/conditional.c
> @@ -411,7 +411,6 @@ static int cond_read_node(struct policydb *p, struct cond_node *node, void *fp)
>                 goto err;
>         return 0;
>  err:
> -       cond_node_destroy(node);
>         return rc;

Since there is now just "return rc" in the error path, can you please
replace all the gotos with plain return statements? And please also
add a Fixes: tag pointing to the commit that introduced the bug (see
Stephen's reply).

Thanks,

-- 
Ondrej Mosnacek
Software Engineer, Platform Security - SELinux kernel
Red Hat, Inc.