linux-kernel - Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

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Date:	Mon, 6 Jun 2011 17:41:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	pageexec@...email.hu
Cc:	Andrew 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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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
Subject: Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls


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

> > I can't see any problem, but exploit writers are exceedingly 
> > clever, and maybe someone has a use for a piece of the code that 
> > isn't a syscall.  Just as a completely artificial example, here's 
> > some buggy code:
> 
> what you're describing here is a classical ret2libc (in modern 
> marketing speak, ROP) attack. in general, having an executable ret 
> insn (with an optional pop even) at a fixed address is very useful, 
> especially for the all too classical case of stack overflows where 
> the attacker may already know of a 'good' function pointer 
> somewhere on the stack but in order to have the cpu reach it, he 
> needs to pop enough bytes off of it. guess what they'll use this 
> ret at a fixed address for...

Good point and i agree that we should get rid of the RETQ there. The 
do_intcc() code can fetch the return address without much fuss - this 
is much faster than doing a #PF.

Please keep reviewing these patches, the security-technical aspects 
of your reviews are extremely useful.

> imho, moving everything to and executing from the vdso page is the 
> only viable solution if you really want to fix the security aspect 
> of the vsyscall mess. it's worked fine for PaX for years now ;).

FYI, this probably means that no-one ever benchmared postgresql 
scalability on a PaX kernel i suspect? Past versions of postgresql 
would big time if you drive the vsyscall time() through through a
#PF ...

Thanks,

	Ingo
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