From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [RFC 4/4] HVM x86 deprivileged mode: Trap handlers for deprivileged mode Date: Fri, 7 Aug 2015 14:19:28 +0100 Message-ID: <55C4B060.7030607@citrix.com> References: <1438879519-564-1-git-send-email-Ben.Catterall@citrix.com> <1438879519-564-5-git-send-email-Ben.Catterall@citrix.com> <55C3D07E.7090701@citrix.com> <55C4A55C.6010400@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55C4A55C.6010400@citrix.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: Ben Catterall , xen-devel@lists.xensource.com Cc: george.dunlap@eu.citrix.com, tim@xen.org, keir@xen.org, ian.campbell@citrix.com, jbeulich@suse.com List-Id: xen-devel@lists.xenproject.org On 07/08/15 13:32, Ben Catterall wrote: > > > On 06/08/15 22:24, Andrew Cooper wrote: >> On 06/08/2015 17:45, Ben Catterall wrote: >>> Added trap handlers to catch exceptions such as a page fault, general >>> protection fault, etc. These handlers will crash the domain as such >>> exceptions >>> would indicate that either there is a bug in deprivileged mode or it >>> has been >>> compromised by an attacker. >>> >>> Signed-off-by: Ben Catterall >>> --- >>> xen/arch/x86/mm/hap/hap.c | 9 +++++++++ >>> xen/arch/x86/traps.c | 41 >>> ++++++++++++++++++++++++++++++++++++++++- >>> 2 files changed, 49 insertions(+), 1 deletion(-) >>> >>> diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c >>> index abc5113..43bde89 100644 >>> --- a/xen/arch/x86/mm/hap/hap.c >>> +++ b/xen/arch/x86/mm/hap/hap.c >>> @@ -685,8 +685,17 @@ static int hap_page_fault(struct vcpu *v, >>> unsigned long va, >>> { >>> struct domain *d = v->domain; >>> + /* If we get a page fault whilst in HVM security user mode */ >>> + if( v->user_mode == 1 ) >>> + { >>> + printk("HVM: #PF (%u:%u) whilst in user mode\n", >>> + d->domain_id, v->vcpu_id); >> %pv is your friend. Like Linux, we have custom printk formats. In this >> case, passing 'v' as a parameter to %pv will cause d$Xv$Y to be >> printed. (The example below predates %pv being introduced). > ok, will do. thanks! >> >>> + domain_crash_synchronous(); >> No need for _synchronous() here. _synchronous() should only be used >> when you can't safely recover. It ends up spinning in a tight loop >> waiting for the next timer interrupt, is anything up to 30ms away. > I'm not sure if we can safely recover from this. This will only be > triggered if there is a bug in depriv mode > or if the mode has been compromised and an attacker has tried to > access unavailable memory. > From my understanding (am I missing something?): domain_crash > effectively sets flags to tell the scheduler that > it should be killed the next time the scheduler runs and then returns. > In which case, if we don't do a > synchronous crash, this return path would return back into the > deprivileged mode, we would not > have mapped in the page (as we shouldn't), and then we get another fault. > > What do you think is the best way forward? Thanks! > Given that there is a use of domain_crash(d) in context below, it is clearly safe to use from here. (Although my general point about hap vs shadow code still applies, meaning that hap_page_fault() is not the correct function to hook like this.) domain_crash() sets a flag, but exiting out from a fault handler heading back towards ring3 code should check for pending softirqs. However, because of the way you have hooked return-to-depriv, you might have broken this. ~Andrew