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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Wu, Feng" <feng.wu@intel.com>, Jan Beulich <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: "keir@xen.org" <keir@xen.org>
Subject: Re: [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV guest code
Date: Wed, 9 Mar 2016 13:42:41 +0000	[thread overview]
Message-ID: <56E02851.4060904@citrix.com> (raw)
In-Reply-To: <E959C4978C3B6342920538CF579893F00C36A634@SHSMSX104.ccr.corp.intel.com>

On 09/03/16 13:35, Wu, Feng wrote:
>
>> -----Original Message-----
>> From: Jan Beulich [mailto:jbeulich@suse.com]
>> Sent: Wednesday, March 9, 2016 8:37 PM
>> To: andrew.cooper3@citrix.com; Wu, Feng <feng.wu@intel.com>; xen-
>> devel@lists.xenproject.org
>> Cc: keir@xen.org
>> Subject: Re: [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV
>> guest code
>>
>>>>>> Andrew Cooper <andrew.cooper3@citrix.com> 03/09/16 1:33 PM >>>
>>> On 09/03/16 12:27, Wu, Feng wrote:
>>>> Oh, thanks for the clarification! Do you know how "An NMI or #MC may
>> occur
>>>> between clearing CR4.SMEP and CR4.SMAP in compat_restore_all_guest and
>>>> it actually returning to guest context, in which case the guest would run with
>>>> the two features enabled. " can happen? Especially how the guest can run
>>>> with the two features enabled?
>>> NMIs and MCEs can occur at any point, even if interrupts are disabled.
>>>
>>> The bad situation is this sequence:
>>>
>>> * Xen is returning to the guest and disables CR4.SMEP/SMAP
>>> * NMI occurs while still in Xen
>>> * NMI exit path sees it is returning to Xen and re-enabled CR4.SMEP/SMAP
>> Well, almost: Re-enabling happens on the NMI entry path. The NMI exit
>> path would, seeing it's returning to Xen context, simply not disable them
>> again.
> Thinking about this again, in this case, when the NMI happens, we are in
> Xen context (CPL in cs is 0), so the CPL of the saved cs in stack is 0,right?
> why do we re-enable CR4.SMEP/SMAP in this case? I mean do we only
> need to enable SMEP/SMAP when coming from 32bit pv guest (CPL of cs is 1) ?

We always want Xen to be running with SMEP/SMAP enabled.  Therefore the
safer and simpler option is to always enable them if we observe them
disabled.

Interrupting a 32bit PV guest might end up seeing a cpl of 1 or 3, and
peeking into the active struct domain to check is_pv32_domain would be a
larger overhead on the entry paths.

~Andrew

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  reply	other threads:[~2016-03-09 13:42 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-04 11:08 [PATCH 0/4] x86: accommodate 32-bit PV guests with SMAP/SMEP handling Jan Beulich
2016-03-04 11:27 ` [PATCH 1/4] x86/alternatives: correct near branch check Jan Beulich
2016-03-07 15:43   ` Andrew Cooper
2016-03-07 15:56     ` Jan Beulich
2016-03-07 16:11       ` Andrew Cooper
2016-03-07 16:21         ` Jan Beulich
2016-03-08 17:33           ` Andrew Cooper
2016-03-04 11:27 ` [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV guest code Jan Beulich
2016-03-07 16:59   ` Andrew Cooper
2016-03-08  7:57     ` Jan Beulich
2016-03-09  8:09       ` Wu, Feng
2016-03-09 14:09         ` Jan Beulich
2016-03-09 11:19       ` Andrew Cooper
2016-03-09 14:28         ` Jan Beulich
2016-03-09  8:09   ` Wu, Feng
2016-03-09 10:45     ` Andrew Cooper
2016-03-09 12:27       ` Wu, Feng
2016-03-09 12:33         ` Andrew Cooper
2016-03-09 12:36           ` Jan Beulich
2016-03-09 12:54             ` Wu, Feng
2016-03-09 13:35             ` Wu, Feng
2016-03-09 13:42               ` Andrew Cooper [this message]
2016-03-09 14:03       ` Jan Beulich
2016-03-09 14:07     ` Jan Beulich
2016-03-04 11:28 ` [PATCH 3/4] x86: use optimal NOPs to fill the SMAP/SMEP placeholders Jan Beulich
2016-03-07 17:43   ` Andrew Cooper
2016-03-08  8:02     ` Jan Beulich
2016-03-04 11:29 ` [PATCH 4/4] x86: use 32-bit loads for 32-bit PV guest state reload Jan Beulich
2016-03-07 17:45   ` Andrew Cooper
2016-03-10  9:44 ` [PATCH v2 0/3] x86: accommodate 32-bit PV guests with SMEP/SMAP handling Jan Beulich
2016-03-10  9:53   ` [PATCH v2 1/3] x86: suppress SMEP and SMAP while running 32-bit PV guest code Jan Beulich
2016-05-13 15:48     ` Andrew Cooper
2016-03-10  9:54   ` [PATCH v2 2/3] x86: use optimal NOPs to fill the SMEP/SMAP placeholders Jan Beulich
2016-05-13 15:49     ` Andrew Cooper
2016-03-10  9:55   ` [PATCH v2 3/3] x86: use 32-bit loads for 32-bit PV guest state reload Jan Beulich
     [not found]   ` <56E9A0DB02000078000DD54C@prv-mh.provo.novell.com>
2016-03-17  7:50     ` [PATCH v3 0/4] x86: accommodate 32-bit PV guests with SMEP/SMAP handling Jan Beulich
2016-03-17  8:02       ` [PATCH v3 1/4] x86: move cached CR4 value to struct cpu_info Jan Beulich
2016-03-17 16:20         ` Andrew Cooper
2016-03-17  8:03       ` [PATCH v3 2/4] x86: suppress SMEP and SMAP while running 32-bit PV guest code Jan Beulich
2016-03-25 18:01         ` Konrad Rzeszutek Wilk
2016-03-29  6:55           ` Jan Beulich
2016-05-13 15:58         ` Andrew Cooper
2016-03-17  8:03       ` [PATCH v3 3/4] x86: use optimal NOPs to fill the SMEP/SMAP placeholders Jan Beulich
2016-05-13 15:57         ` Andrew Cooper
2016-05-13 16:06           ` Jan Beulich
2016-05-13 16:09             ` Andrew Cooper
2016-03-17  8:04       ` [PATCH v3 4/4] x86: use 32-bit loads for 32-bit PV guest state reload Jan Beulich
2016-03-25 18:02         ` Konrad Rzeszutek Wilk
2016-03-17 16:14       ` [PATCH v3 5/4] x86: reduce code size of struct cpu_info member accesses Jan Beulich
2016-03-25 18:47         ` Konrad Rzeszutek Wilk
2016-03-29  6:59           ` Jan Beulich
2016-03-30 14:28             ` Konrad Rzeszutek Wilk
2016-03-30 14:42               ` Jan Beulich
2016-05-13 16:11         ` Andrew Cooper
2016-05-03 13:58       ` Ping: [PATCH v3 2/4] x86: suppress SMEP and SMAP while running 32-bit PV guest code Jan Beulich
2016-05-03 14:10         ` Andrew Cooper
2016-05-03 14:25           ` Jan Beulich
2016-05-04 10:03             ` Andrew Cooper
2016-05-04 13:35               ` Jan Beulich
2016-05-04  3:07         ` Wu, Feng
2016-05-13 15:21         ` Wei Liu
2016-05-13 15:30           ` Jan Beulich
2016-05-13 15:33             ` Wei Liu
2016-05-13 17:02       ` [PATCH v3 0/4] x86: accommodate 32-bit PV guests with SMEP/SMAP handling Wei Liu
2016-05-13 17:21         ` Andrew Cooper
2016-06-21  6:19       ` Wu, Feng
2016-06-21  7:17         ` Jan Beulich

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