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From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.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 05:00:15 -0600	[thread overview]
Message-ID: <56EA9C5002000078000DDBBE@prv-mh.provo.novell.com> (raw)
In-Reply-To: <56EA8ACC.2080102@citrix.com>

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

Jan


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  reply	other threads:[~2016-03-17 11:00 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 [this message]
2016-03-17 11:05         ` Andrew Cooper

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