linux-kernel - Re: [PATCH 2.6.29.6 - 2.6.35-rc3] CRED: Release spinlock before commit

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Message-ID: <21796.1276875943@redhat.com>
Date:	Fri, 18 Jun 2010 16:45:43 +0100
From:	David Howells <dhowells@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH 2.6.29.6 - 2.6.35-rc3] CRED: Release spinlock before commit_creds().

Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:

> reparent_to_kthreadd() is calling commit_creds() with tasklist_lock held.
> But commit_creds() calls
> key_fsuid_changed()/key_fsgid_changed()/proc_id_connector() which may sleep.
> Release tasklist_lock before calling commit_creds().

Hmmm...  Does this change leave a hole in which the thread can be accessed as
it's not locked away, but still has the old credentials?

I wonder if we should have an internal CLONE_KTHREAD flag to achieve this in
do_fork() rather than daemonize()'ing the thread later.

Also, key_fsuid_changed() and key_fsgid_changed() should be skipped in
commit_creds() if the thread-keyring pointer in the new creds is not the same
as that in the old creds (as will be the case if we're shifting to init_cred),
so they shouldn't really be a problem.

However, I can't see an easy way around proc_id_connector() sleeping - unless
we just don't call it if new == &init_cred in commit_creds().

I suppose it also leaves a hole if we call commit_creds() first, before
getting the write_lock()... :-/

David
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