linux-hardening - Re: Limited/Broken functionality of ASLR for Libs >= 2MB

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Message-ID: <ZaWZqnD7XmP5HMA0@casper.infradead.org>
Date: Mon, 15 Jan 2024 20:46:34 +0000
From: Matthew Wilcox <willy@...radead.org>
To: mail@...otw.com
Cc: linux-hardening@...r.kernel.org, Jakub Wilk <jwilk@...lk.net>,
	Salvatore Bonaccorso <carnil@...ian.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	William Kucharski <william.kucharski@...cle.com>
Subject: Re: Limited/Broken functionality of ASLR for Libs >= 2MB

On Mon, Jan 15, 2024 at 07:21:19PM +0100, mail@...otw.com wrote:
> Am 15.01.2024 17:52, schrieb Matthew Wilcox:
> > On Mon, Jan 15, 2024 at 04:40:36PM +0000, Sam James wrote:
> > > mail@...otw.com writes:
> > > > Hey, I read that ASLR is currently (since kernel >=5.18) broken for
> > > > 32bit libs and reduced in effectiveness for 64bit libs... (the issue
> > > > only arises if a lib is over 2MB).
> > > > I confirmed this for myself but only for the 64bit case.
> > > >
> > > > I saw that this issue is being tracked by ubuntu
> > > > (https://bugs.launchpad.net/ubuntu-kernel-tests/+bug/1983357).
> > > > If this is the wrong place and I should instead report it elsewhere I
> > > > am very sorry.
> > > 
> > > See also https://bugs.debian.org/1024149. Unfortunately, I don't
> > > think the issue found its way upstream until now (thanks).
> > > 
> > > CCing relevant maintainers (per the Debian bug).
> > 
> > You know, my email address is all over that commit and the doofus who
> > "discovered the vulnerability" didn't even have the courtesy to let
> > me know.  I've had several private emails about this over the last few
> > days and I just don't care.  Who's running 32-bit code and cares about
> > security?  32-bit kernels are known-vulnerable to all kinds of security
> > problems, and I think this is the least of your worries.
> > 
> > This was intended to happen, it's not a surprise.
> 
> Hi,
> first of all I am very sorry, I didn't realize I should have contacted you
> first (I'm not the one who found the bug initially), I will do it
> differently in the future.

I'm not annoyed *at you*.  I'm annoyed at the guy who first "discovered"
it.  I'm annoyed at the people who are running around with their hair
on fire.  I'm annoyed at all the people who *didn't* contact me.

> Unfortunately, my knowledge is not sufficient to judge how bad it is that
> 32bit effectively has no ASLR support anymore.
> 
> 64bit is also affected, even though there are probably more than enough
> bits left there? I have since seen that both Arch and Ubuntu seem to have
> "patches" in place (https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/commit/3904bcb32cc58c10232fb618bf96c1b43b0bc9d7)
> in which they set the `CONFIG_ARCH_MMAP_RND_BITS=32` and
> `CONFIG_ARCH_MMAP_RND_COMPAT_BITS=16`, I'm not sure if this is a good
> result or if it will cause other problems.

Yeah, I don't know either.  Outside my scope of expertise.

I received a suggestion off-list that we only do the PMD alignment on
64-bit, which seems quite reasonable to me.  After all, I don't care
about performance on 32-bit just as much as I don't care about security
on 32-bit.