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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Xen-devel <xen-devel@lists.xen.org>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH 2/2] xen/x86: Introduce a new VMASSIST for architectural behaviour of iopl
Date: Thu, 17 Mar 2016 11:05:26 +0000	[thread overview]
Message-ID: <56EA8F76.7070600@citrix.com> (raw)
In-Reply-To: <56EA9C5002000078000DDBBE@prv-mh.provo.novell.com>

On 17/03/16 11:00, Jan Beulich wrote:
>>>> On 17.03.16 at 11:45, <andrew.cooper3@citrix.com> wrote:
>> On 17/03/16 10:25, Jan Beulich wrote:
>>>>>> On 16.03.16 at 21:05, <andrew.cooper3@citrix.com> wrote:
>>>> @@ -1742,8 +1742,10 @@ static void load_segments(struct vcpu *n)
>>>>              cs_and_mask = (unsigned short)regs->cs |
>>>>                  ((unsigned int)vcpu_info(n, evtchn_upcall_mask) << 16);
>>>>              /* Fold upcall mask into RFLAGS.IF. */
>>>> -            eflags  = regs->_eflags & ~X86_EFLAGS_IF;
>>>> +            eflags  = regs->_eflags & ~(X86_EFLAGS_IF|X86_EFLAGS_IOPL);
>>> This and ...
>>>
>>>> @@ -1788,8 +1790,10 @@ static void load_segments(struct vcpu *n)
>>>>              ((unsigned long)vcpu_info(n, evtchn_upcall_mask) << 32);
>>>>  
>>>>          /* Fold upcall mask into RFLAGS.IF. */
>>>> -        rflags  = regs->rflags & ~X86_EFLAGS_IF;
>>>> +        rflags  = regs->rflags & ~(X86_EFLAGS_IF|X86_EFLAGS_IOPL);
>>> ... this is not really necessary (but also not wrong) - the actual
>>> EFLAGS.IOPL is always zero (and assumed to be so by code
>>> further down from the respective adjustments you make). For
>>> consistency's sake it might be better to either drop the changes
>>> here, or also adjust the two places masking regs->eflags.
>> I will adjust the others.  I would prefer not to rely on the assumption
>> that it is actually 0.
> But you realize that if it wasn't zero, we'd have a security issue?

Indeed.  But as this adjustment is literally free for us to use, making
Xen a little more robust in the (hopefully never) case were IOPL ends up
not being 0.

~Andrew

> (This notwithstanding I'm fine with both directions, as indicated
> before.)
>
> Jan
>


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      reply	other threads:[~2016-03-17 11:05 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-16 20:05 [PATCH 0/2] XSA-171 Followup work Andrew Cooper
2016-03-16 20:05 ` [PATCH 1/2] xen/x86: Don't hold TRAPBOUNCE_flags in %cl during create_bounce_frame Andrew Cooper
2016-03-16 20:05 ` [PATCH 2/2] xen/x86: Introduce a new VMASSIST for architectural behaviour of iopl Andrew Cooper
2016-03-17 10:25   ` Jan Beulich
2016-03-17 10:45     ` Andrew Cooper
2016-03-17 11:00       ` Jan Beulich
2016-03-17 11:05         ` Andrew Cooper [this message]

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