[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH v12 1/9] x86: add generic resource (e.g. MSR) access hypercall

On 07/08/2014 11:07 AM, Andrew Cooper wrote:
On 08/07/14 08:06, Xu, Dongxiao wrote:
-----Original Message-----
From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
Sent: Friday, July 04, 2014 6:53 PM
To: Jan Beulich; Xu, Dongxiao; xen-devel@xxxxxxxxxxxxx
Cc: Ian.Campbell@xxxxxxxxxx; George.Dunlap@xxxxxxxxxxxxx;
Ian.Jackson@xxxxxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx;
konrad.wilk@xxxxxxxxxx; dgdegra@xxxxxxxxxxxxx; keir@xxxxxxx
Subject: Re: [PATCH v12 1/9] x86: add generic resource (e.g. MSR) access

On 04/07/14 11:30, Jan Beulich wrote:
On 04.07.14 at 11:40, <andrew.cooper3@xxxxxxxxxx> wrote:
On 04/07/14 09:34, Dongxiao Xu wrote:
Add a generic resource access hypercall for tool stack or other
components, e.g., accessing MSR, port I/O, etc.

Signed-off-by: Dongxiao Xu <dongxiao.xu@xxxxxxxxx>
This still permits a user of the hypercalls to play with EFER or
SYSENTER_EIP, which obviously is a very bad thing.

There needs to be a whitelist of permitted MSRs which can be accessed.
Hmm, I'm not sure. One particular purpose I see here is to allow the
tool stack (or Dom0) access to MSRs Xen may not know about (yet).
Furthermore, this being a platform op, only the hardware domain
should ever have access, and it certainly ought to know what it's
doing. So the sum of these two considerations is: If at all, we may
want a black list here.


I don't think it is safe for the toolstack to ever be playing with MSRs
which Xen is completely unaware of.  There is no guarentee whatsoever
that a new MSR which Xen is unaware of doesn't have security
implications if the toolstack were to play with it.

Adding entries to a whitelist is easy and could be considered a
maintenance activity similar to keeping the model/stepping information
This resource access mechanism is useful according to some conversation with 
IPDC customers. Per their description, once the machine and VMs are online, 
they will not be turned off. Sometimes administrators may need to dynamically 
modify some resource values to fix/workaround certain issues, and our patch may 
serve this purpose.

Adding the white/black list will bring certain constraints for the above use 
case. Also considering that the tool stack resides in dom0, I think it is not 
so dangerous.

The whole purpose of XSM is to split the toolstack up so it isn't all in

Extending a whitelist is trivial, and requires a positive action on
behalf of someone to decide that the added MSR *is* safe.  Anything else
is a security bug waiting to happen.

Why not adding a whitelist which is tested as default and make the test
switchable via a boot parameter?


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.