linux-kernel - Re: [PATCH] binfmt_elf: fix PIE load with randomization disabled

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Message-ID: <alpine.LNX.2.00.1312092210280.24835@pobox.suse.cz>
Date:	Mon, 9 Dec 2013 22:13:45 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"H.J. Lu" <hjl.tools@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Josh Boyer <jwboyer@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] binfmt_elf: fix PIE load with randomization disabled

On Mon, 9 Dec 2013, H.J. Lu wrote:

> >> Normally, a PIE executable has zero virtual address on the first PT_LOAD
> >> segment and kernel will load such executable at random address when
> >> randomization is enabled.  If randomization is disabled, kernel will load
> >> it at a fixed address.  But if a PIE executable has non-zero virtual
> >> address on the first PT_LOAD segment, kernel will load such executable
> >> at the non-zero virtual address when randomization is enabled. But when
> >> randomization is disabled, kernel ignores the non-zero virtual address
> >> at the non-zero virtual address when randomization is enabled.
> >
> > Hmm ... isn't actually this the thing that needs to be fixed instead?
> >
> > IOW, when randomization is enabled, is there a reason not to load on
> > randomized address? (even if the first PT_LOAD segment has non-zero
> > vaddr?)
> 
> No, please don't do that.  Normally, PIE has zero load address and kernel
> can load it anywhere.  There are multiple reasons why PIE has non-zero
> load address.  Saying you need to load a program above 4GB under x86-64,
> you can't do that with normal dynamic executable.  PIE with non-zero load
> address is the only way to do that on x86-64.

Hmm, so if it's because of 4G PT_LOAD limit, how about at least adding 
randomized offset to the supplied vaddr?

PT_LOAD being non-zero causing randomization to be turned off seems like 
quite unexpected behavior to me, with a great potential to cause a lot of 
confusion.

-- 
Jiri Kosina
SUSE Labs
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