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Re: [Xen-devel] [PATCH] x86/hvm: Intercept RDPMC when vPMU is disabled
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- Date: Fri, 22 Feb 2019 16:58:27 -0500
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Brian Woods <brian.woods@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 22 Feb 2019 21:59:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 2/22/19 4:13 PM, Andrew Cooper wrote:
> vPMU isn't security supported, and in general guests can't access any of the
> performance counter MSRs. However, the RDPMC instruction isn't intercepted,
> meaning that guest software can read the instantaneous counter values.
>
> When vPMU isn't configured, intercept RDPMC and unconditionally fail it as if
> software has requested a bad counter index (#GP fault). It is model specific
> as to which counters are available to begin with, and in levelled scenarios,
> this information may not be accurate in the first place.
>
> This change isn't expected to have any impact on VMs. Userspace is not
> usually given access to RDPMC (Windows appear to completely prohibit it; Linux
> is restricted to root), and kernels won't be executing RDPMC instructions if
> their PMU drivers have failed to start.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> CC: Brian Woods <brian.woods@xxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
>
> This should be taken into Xen 4.12 and backported to the stable releases.
> While it isn't an XSA itself, it is an information leak (Xen's NMI watchdog in
> particular) which could be advantagous to an attacker trying to exploit a race
> condition.
>
> The only other option is to emulate the reported family and offer back all 0's
> for the accessable counters. Obviously this is a non-starter.
When VPMU is off MSR reads return zero. While it is debatable whether
this the right action, shouldn't rdpmc behave in the same fashion?
-boris
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