[Lucid,CVE-2012-2745] cred: copy_process() should clear child->replacement_session_keyring

From: Oleg Nesterov <oleg@redhat.com>

CVE-2012-2745

BugLink: http://bugs.launchpad.net/bugs/1023535

keyctl_session_to_parent(task) sets ->replacement_session_keyring,
it should be processed and cleared by key_replace_session_keyring().

However, this task can fork before it notices TIF_NOTIFY_RESUME and
the new child gets the bogus ->replacement_session_keyring copied by
dup_task_struct(). This is obviously wrong and, if nothing else, this
leads to put_cred(already_freed_cred).

change copy_creds() to clear this member. If copy_process() fails
before this point the wrong ->replacement_session_keyring doesn't
matter, exit_creds() won't be called.

Cc: <stable@vger.kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(back ported from commit 79549c6dfda0603dba9a70a53467ce62d9335c33)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
---
 kernel/cred.c |    1 +
 1 file changed, 1 insertion(+)

On 09/05/2012 12:35 PM, Tim Gardner wrote:
> From: Oleg Nesterov <oleg@redhat.com>
> 
> CVE-2012-2745
> 
> BugLink: http://bugs.launchpad.net/bugs/1023535
> 
> keyctl_session_to_parent(task) sets ->replacement_session_keyring,
> it should be processed and cleared by key_replace_session_keyring().
> 
> However, this task can fork before it notices TIF_NOTIFY_RESUME and
> the new child gets the bogus ->replacement_session_keyring copied by
> dup_task_struct(). This is obviously wrong and, if nothing else, this
> leads to put_cred(already_freed_cred).
> 
> change copy_creds() to clear this member. If copy_process() fails
> before this point the wrong ->replacement_session_keyring doesn't
> matter, exit_creds() won't be called.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Acked-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> (back ported from commit 79549c6dfda0603dba9a70a53467ce62d9335c33)
> 
> Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
> ---
>  kernel/cred.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 0b5b5fc..6ffd433 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -442,6 +442,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
>  	int ret;
>  
>  	mutex_init(&p->cred_guard_mutex);
> +	p->replacement_session_keyring = NULL;
>  
>  	if (
>  #ifdef CONFIG_KEYS
>