From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option Date: Wed, 29 Jul 2015 20:03:07 +0100 Message-ID: <55B9236B.9090507__31264.3962483437$1438196757$gmane$org@citrix.com> References: <55B64FEA.70204@oracle.com> <55B659EC.5030009@oracle.com> <55B75993.90909@citrix.com> <55B7AE39.7000101@citrix.com> <55B7B791.2050208@oracle.com> <55B822B8.3090608@citrix.com> <55B841FF.2000102@oracle.com> <55B8E16C.2050406@citrix.com> <55B8E68B.2030305@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55B8E68B.2030305@oracle.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: Boris Ostrovsky , Andy Lutomirski Cc: "security@kernel.org" , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , Borislav Petkov , David Vrabel , Jan Beulich , Sasha Levin List-Id: xen-devel@lists.xenproject.org On 29/07/15 15:43, Boris Ostrovsky wrote: > FYI, I have got a repro now and am investigating. Good and bad news. This bug has nothing to do with LDTs themselves. I have worked out what is going on, but this: diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c index 5abeaac..7e1a82e 100644 --- a/arch/x86/xen/enlighten.c +++ b/arch/x86/xen/enlighten.c @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v, pgprot_t prot) pte = pfn_pte(pfn, prot); + (void)*(volatile int*)v; if (HYPERVISOR_update_va_mapping((unsigned long)v, pte, 0)) { pr_err("set_aliased_prot va update failed w/ lazy mode %u\n", paravirt_get_lazy_mode()); BUG(); Is perhaps not the fix we are looking for, and every use of HYPERVISOR_update_va_mapping() is susceptible to the same problem. The update_va_mapping hypercall is designed to emulate writing the pte for v, with auditing applied. As part of this, it does a pagewalk on v to locate and map the l1. During this walk, Xen it finds the l2 not present, and fails the hypercall. i.e. v is not reachable from the current cr3. Reading the virtual address immediately before issuing the hypercall causes Linux's memory faulting logic to fault in the l2. This also explains why vm_unmap_aliases() appears to fix the issue; it is likely to fault in enough of the paging structure for v to be reachable. One solution might be to use MMU_NORMAL_PT_UPDATE hypercall instead, which take the physical address of pte to update. This won't fail in Xen if part of the paging structure is missing, and can be batched. ~Andrew