linux-kernel - Re: [patch] remove MNT_NOEXEC check for PROT

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Date:	Sat, 23 Sep 2006 19:42:03 +0400
From:	Stas Sergeev <stsp@...et.ru>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...l.org>, Ulrich Drepper <drepper@...hat.com>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] remove MNT_NOEXEC check for PROT_EXEC mmaps

Hi.

Hugh Dickins wrote:
> automatically.  But they were put in for good reason, have been
> in for nearly three years, I doubt they should come out now.
I know they won't. I only thought I have to try, after seeing
the debian problem and googling out a few negative posts about
these checks.

> It's hardly any surprise, is it, that if a distro chooses now
> to mount something "noexec", a problem is then found with a few
> things which want otherwise?
They do not "want otherwise". They do the right thing - use
shm_open() and then mmap(), but mmap() suddenly fails. The apps
are not guilty. Neither I think the debian guys are.

> And it seems unlikely that the answer
> is then to modify the kernel, to weaken the very protection they're
> wanting to add?
I don't think they want to prevent PROT_EXEC mmaps. Almost
certainly not. Maybe they thought they would only block mere
execve() calls and the like, I don't know. My point is that
this change (use of "noexec") should not break the properly
written apps, but right now it does. Is it stated anywhere
in the shm_open() manpage or elsewhere that you must not use
"noexec" on tmpfs or you'll get troubles with mmap?

> The original 2.6.0 patch (later backported into 2.4.25) was
> <drepper@...hat.com>
> 	[PATCH] Fix 'noexec' behaviour
> 	We should not allow mmap() with PROT_EXEC on mounts marked "noexec",
> 	since otherwise there is no way for user-supplied executable loaders
> 	(like ld.so and emulator environments) to properly honour the
> 	"noexec"ness of the target.
Thanks for the pointer, but that looks like the user-space
issue to me. Why ld.so can't figure out the "noexecness" and
do the right thing itself? Or does it figure out the "noexecness"
exactly by trying the PROT_EXEC mmap and see if it fails?

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