linux-kernel - Re: [PATCH] x86-64, vsyscalls: Rename UNSAFE_VSYSCALLS to COMPAT

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Date:	Mon, 6 Jun 2011 14:47:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	pageexec@...email.hu
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@....edu>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Jesper Juhl <jj@...osbits.net>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>,
	Brian Gerst <brgerst@...il.com>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Valdis.Kletnieks@...edu, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86-64, vsyscalls: Rename UNSAFE_VSYSCALLS to
 COMPAT_VSYSCALLS

* pageexec@...email.hu <pageexec@...email.hu> wrote:

> On 6 Jun 2011 at 12:24, Ingo Molnar wrote:
> 
> > 
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > 
> > > On Mon, Jun 6, 2011 at 2:50 AM, Andy Lutomirski <luto@....edu> wrote:
> > > > CONFIG_UNSAFE_VSYSCALLS was added in the previous patch as a
> > > > temporary hack to avoid penalizing users who don't build glibc from
> > > > git.
> > > 
> > > I really hate that name.
> > > 
> > > Do you have *any* reason to call this "unsafe"?
> > 
> > No, there's no reason at all for that. That naming is borderline 
> > security FUD and last time i saw the series i considered renaming
> > it but got distracted :-)
> 
> security FUD? for real? ;) [...]

'Borderline' security FUD! :-)

> [...] does that mean that you guys would accept a patch that would 
> map the vdso at a fixed address for old times's sake? if not, on 
> what grounds would you refuse it? see, you can't have it both ways.

You can actually do that by enabling CONFIG_COMPAT_VDSO=y.

> the fixed address of the vsyscall page *is* a very real security 
> problem, it should have never been accepted as such and it's high 
> time it went away finally in 2011AD.

It's only a security problem if there's a security hole elsewhere.

The thing is, and i'm not sure whether you realize or recognize it, 
but these measures *are* two-edged swords.

Yes, the upside is that they reduce the risks associated with 
security holes - but only statistically so.

The downside is that having such a measure in place makes it somewhat 
less likely that those bugs will be found and fixed in the future: if 
a bug is not exploitable then people like Spender wont spend time 
exploiting and making a big deal out of them, right?

And yes, it might be embarrasing to see easy exploits and we might 
roll eyes at the associated self-promotion circus but it will be one 
more bug found, the reasons for the bug will be examined, potentially 
avoiding a whole class of similar bugs *for sure*.

Can you guarantee that security bugs will be found and fixed with the 
same kind of intensity even if we make their exploitation (much) 
harder? I don't think you can make such a guarantee.

So as long as we are trading bugs-fixed-for-sure against statistical 
safety we have to be mindful of the downsides of such a tradeoff ...

Thanks,

	Ingo
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