linux-kernel - Re: commit

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Message-ID: <87vc9b61q2.fsf@xmission.com>
Date:	Thu, 28 Feb 2013 23:03:33 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Jones <davej@...hat.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>, dhowells@...hat.com,
	"Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: commit_creds oops

Dave Jones <davej@...hat.com> writes:

> On Thu, Feb 28, 2013 at 04:25:40PM -0800, Eric W. Biederman wrote:
>  
>  > > [   89.639850] RIP: 0010:[<ffffffff810784b0>]  [<ffffffff810784b0>] commit_creds+0x250/0x2f0
>  > > [   89.658399] Call Trace:
>  > > [   89.658822]  [<ffffffff812c7d9b>] key_change_session_keyring+0xfb/0x140
>  > > [   89.659845]  [<ffffffff8106c665>] task_work_run+0xa5/0xd0
>  > > [   89.660698]  [<ffffffff81002911>] do_notify_resume+0x71/0xb0
>  > > [   89.661581]  [<ffffffff816c9a4a>] int_signal+0x12/0x17
>  > >
>  > > Appears to be..
>  > >
>  > >                 if ((set_ns == subset_ns->parent)  &&
>  > >      850:       48 8b 8a c8 00 00 00    mov    0xc8(%rdx),%rcx
>  > >
>  > > from the inlined cred_cap_issubset
>  > 
>  > Interesting.  That line is protected with the check subset_ns !=
>  > &init_user_ns so subset_ns->parent must be valid or subset_ns is not
>  > a proper user namespace.
>  > 
>  > Ugh.  I think I see what is going on and it is just silly. 
>  > 
>  > It looks like by historical accident we have been reading trying to set
>  > new->user_ns from new->user_ns.  Which is totally silly as new->user_ns
>  > is NULL (as is every other field in new except session_keyring at that
>  > point).
>  > 
>  > It looks like it is safe to sleep in key_change_session_keyring so why
>  > we just don't use prepare_creds there like everywhere else is beyond
>  > me.
>  > 
>  > The intent is clearly to copy all of the fields from old to new so what
>  > we should be doing is is copying old->user_ns into new->user_ns.
>  > 
>  > Dave can you verify that this patch fixes the oops?
>
> Looks like it.  Haven't hit the same thing since applying your patch.
>
> I noticed though that get_user_ns bumps a refcount.  Is this what we
> want if we're just copying ?

Yes.  commit_creds(new) winds up finding old on the current process
and calling put_cred(old).

put_cred when the count drops to zero winds up calling put_cred_rcu
which calls put_user_ns(old->user_ns);

For the same reason we need an extra count on the user namespace new
so that when it eventually is put and put_user_ns(new->user_ns) is
called we don't have a negative count.

Which is a long of way of saying yes we are adding another reference and
we need to increase the reference count.

Eric
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