linux-kernel - Re: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls

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Message-ID: <20080222124950.GA81921@muc.de>
Date:	Fri, 22 Feb 2008 13:49:50 +0100
From:	Andi Kleen <ak@....de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Arne Georg Gleditsch <arne.gleditsch@...phinics.no>,
	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls

On Thu, Feb 21, 2008 at 08:45:53PM +0100, Thomas Gleixner wrote:
> On Thu, 21 Feb 2008, Andi Kleen wrote:
> 
> > On Wed, Feb 20, 2008 at 02:57:34PM +0100, Arne Georg Gleditsch wrote:
> > > Hi,
> > > 
> > > I'm looking at 2.6.25-rc2.  vsyscall_sysctl_change contains code to NOP
> > > out the actual system call instructions of the vsyscall page when
> > > vsyscall64 is enabled.  This seems to interact badly with the fallback
> > > code in do_vgettimeofday which tries to call gettimeofday if the
> > > configured clock source does not support vread.  (In effect,
> > > gettimeofday() becomes a nop and time() always returns 0.  Not very
> > > useful.)
> > > 
> > > Is there a good reason to keep this?  Aren't the instructions in
> > > question avoided (or invoked) according to the vsyscall64 flag by the
> > > surrounding logic anyway?
> > 
> > Yes they are.  But a system call sequence at a known fixed address
> > is potentially useful to exploits. That is why it is nop'ed out when
> > it is not needed.
> 
> That's a nice intent, but the reality is that this code is broken as
> hell:

Well it worked when I wrote it, but it's quite possible it didn't survive
the clocksource conversion completely.

-Andi
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