linux-kernel - Re: [KVM paravirt issue?] Re: vsyscall=emulate regression

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Date:	Thu, 16 Feb 2012 19:14:03 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Amit Shah <amit.shah@...hat.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>
Subject: Re: [KVM paravirt issue?] Re: vsyscall=emulate regression

On 02/16/2012 06:45 PM, Andy Lutomirski wrote:
> >
> >> So I could have messed up, or there could be a subtle
> >> bug somewhere.  Any ideas?
> >
> > What's the code trying to do?  Execute an instruction from an
> > non-executable page, trap the #PF, and emulate?  And what are the
> > symptoms? wrong error code for the #PF?  That could easily be a kvm bug.
> >
>
> The symptom is that some kind of access to a page that's supposed to
> be readable, NX is reporting error 5.  I'm not quite sure what kind of
> access is causing that.

Might it be a fetch access, with kvm forgetting to set bit 4 correctly?

> >
> > Can you point me at the code in question?
>
> The setup code is in arch/x86/kernel/vsyscall_64.c in map_vsyscall.
> The bad access is to the vsyscall page.

The bad access is on purpose, yes?

>From fault.c:

#ifdef CONFIG_X86_64
                /*
                 * Instruction fetch faults in the vsyscall page might need
                 * emulation.
                 */
                if (unlikely((error_code & PF_INSTR) &&
                             ((address & ~0xfff) == VSYSCALL_START))) {
                        if (emulate_vsyscall(regs, address))
                                return;
                }
#endif

so it seems like kvm doesn't set PF_INSTR?

I thought we unit tested that, but maybe not this exact scenario.


-- 
error compiling committee.c: too many arguments to function

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