linux-kernel - Re: [PATCH] KVM: nVMX: VMX instructions: fix segment checks when L1 is in long mode.

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Date:	Wed, 29 Jun 2016 19:25:54 +0200
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	x86 <x86@...nel.org>, kvm <kvm@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Eugene Korenevsky <ekorenevsky@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH] KVM: nVMX: VMX instructions: fix segment checks when L1
 is in long mode.

On Fri, Jun 24, 2016 at 03:10:03PM +0200, Paolo Bonzini wrote:
> On 24/06/2016 15:04, Quentin Casasnovas wrote:
> > On Thu, Jun 23, 2016 at 06:03:01PM +0200, Paolo Bonzini wrote:
> >>
> >>
> >> On 18/06/2016 11:01, Quentin Casasnovas wrote:
> >>> Cross-checking the KVM/VMX VMREAD emulation code with the Intel Software
> >>> Developper Manual Volume 3C - "VMREAD - Read Field from Virtual-Machine
> >>> Control Structure", I found that we're enforcing that the destination
> >>> operand is NOT located in a read-only data segment or any code segment when
> >>> the L1 is in long mode - BUT that check should only happen when it is in
> >>> protected mode.
> >>>
> >>> Shuffling the code a bit to make our emulation follow the specification
> >>> allows me to boot a Xen dom0 in a nested KVM and start HVM L2 guests
> >>> without problems.
> >>
> >> That's great, and I'm applying the patch, but it's also pretty weird. :)
> >>  Do you have a pointer to Xen source code that does a VMREAD into a
> >> read-only data segment or a code segment?
> > 
> > It is indeed pretty weird.  Looking at the Xen stack trace, it looks like
> > the vmread is writing to an on-stack buffer, and surely it must be writable
> > so I wonder if Xen might not be using an executable stack for some reason?
> > That would be a bit scary so I'm surely missing something.
> > 
> > Is there an easy way to know from my KVM host the different segment
> > permission setup by the guest?
> 
> Remove your patch, call dump_vmcs() where the #GP is injected, and
> you'll find the VMCS (including segment permissions, but not the
> instruction info field---you probably should add it) in dmesg.
> 

Thanks for the heads up :)

I've had a bit more time to spend on this this morning and attached is the
VMCS dump.  I've look at the vmcs_instruction_info and it appears the
segment referenced is SS (which is in sync with the backtrace where the
instruction causing the vmexit is "vmread %rbp, %rbp), and it has awkward
attributes:

  SS:   sel=0x0000, attr=0x1c000, limit=0xffffffff, base=0x0000000000000000

The lower 16 bits are all zero so KVM VMX emulation was injecting the GP(0)
because we were about to write to a read-only segment.  At least the stack
isn't executable from what I can tell!

Attached is the full VMCS dump where I've added a printk() to show the
'type' (all zeroes) and vmcs_instruction_info in case my above analysis is
complete non-sense.

Quentin

View attachment "vmcs_dump_xen_vmread.txt" of type "text/plain" (2901 bytes)