From mboxrd@z Thu Jan 1 00:00:00 1970 From: Razvan Cojocaru Subject: Re: Blocking CR and MSR writes via mem_access? Date: Thu, 02 Oct 2014 14:46:19 +0300 Message-ID: <542D3B0B.9070808@bitdefender.com> References: <542D2DA7.1060903@bitdefender.com> <542D55AE020000780003BEFB@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <542D55AE020000780003BEFB@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: "xen-devel@lists.xen.org" List-Id: xen-devel@lists.xenproject.org On 10/02/14 14:39, Jan Beulich wrote: >>>> On 02.10.14 at 12:49, wrote: >> Currently hvm_memory_event_cr3() and the other hvm_memory_event_*() >> functions in hvm.c can pause the VCPU and send a mem_event with the new >> value of the respective register, but especially in the case of CR >> events (as opposed to MSR events), this is done _after_ the value is set >> (please see hvm_set_cr3() in hvm.c). >> >> It would be interesting from a memory introspection application's point >> of view to be able to receive a mem_event _before_ the value is set, and >> important to be able to veto the change. > > So what do you expect the effect of denying the write to be? > Wouldn't crashing the guest explicitly have about the same effect? Thanks for the quick reply! Denying a normal, legitimate write, would indeed be a problem along the lines of what you are describing, but the point would be to block malicious writes that would modify the SYSCALL entry point, disable SMAP / SMEP, and so on. >> 1. Would it be acceptable to move the CR3 event sending code so that a >> mem_access client would receive the event _before_ the write takes >> place? Is this likely to break other mem_event clients that might rely >> on the event being received _after_ the value has been set? > > I don't think you should break existing behavior. If you need an > event before the setting gets carried out, just add another one. Understood. Thanks, Razvan Cojocaru