linux-kernel - Re: init: How did init/do_mounts_rd.c overcome memory protection ?

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Message-ID: <CAFLxGvw+eR8O+mSBdLURbbjPSpgnzWuSkirwe6cFc_8KzQ444g@mail.gmail.com>
Date:	Tue, 10 Nov 2015 22:37:10 +0100
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Badhri Jagan Sridharan <badhri@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: init: How did init/do_mounts_rd.c overcome memory protection ?

On Tue, Nov 10, 2015 at 7:38 PM, Badhri Jagan Sridharan
<badhri@...gle.com> wrote:
> Mighty upstream,
>
> I see that do_mounts_rd.c seems to make calls to sys_read and
> sys_lseek functions. As these are syscall functions, they expects
> some of the arguments to be from userspace.
>
> I was going through the article that Greg KH wrote a while back:
> http://www.linuxjournal.com/article/8110?page=0,1 . I don't see any
> references to set_fs/get_fs under init/*. Does the memory protection
> get enabled only in the later stage ? Or does do_mounts_rd.c accomplish
> this in some other way ?

The stuff in init/ is PID 1 and it inherits addr_limit from the
initial thread (PID 0 or swapper called).
INIT_THREAD_INFO() sets addr_limit to KERNEL_DS.

-- 
Thanks,
//richard
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