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

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Date:	Thu, 21 Feb 2008 16:58:52 +0100
From:	Andi Kleen <ak@....de>
To:	Arne Georg Gleditsch <arne.gleditsch@...phinics.no>
Cc:	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls

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.

-Andi
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