From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: KVM: x86: check for cr3 validity in ioctl_set_sregs Date: Thu, 16 Apr 2009 15:49:24 +0300 Message-ID: <49E72954.8040005@redhat.com> References: <20090415221042.GA20127@amt.cnet> <49E6F2AF.9050300@redhat.com> <20090416091042.GA6709@amt.cnet> <49E6F92C.2000202@redhat.com> <20090416113044.GA11200@amt.cnet> <49E727D3.8050306@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm To: Marcelo Tosatti Return-path: Received: from mx2.redhat.com ([66.187.237.31]:49261 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752893AbZDPMtb (ORCPT ); Thu, 16 Apr 2009 08:49:31 -0400 Received: from int-mx2.corp.redhat.com (int-mx2.corp.redhat.com [172.16.27.26]) by mx2.redhat.com (8.13.8/8.13.8) with ESMTP id n3GCnVvZ000356 for ; Thu, 16 Apr 2009 08:49:31 -0400 In-Reply-To: <49E727D3.8050306@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Avi Kivity wrote: > Marcelo Tosatti wrote: >> Matt T. Yourst notes that kvm_arch_vcpu_ioctl_set_sregs lacks validity >> checking for the new cr3 value: >> "Userspace callers of KVM_SET_SREGS can pass a bogus value of >> cr3 to >> the kernel. This will trigger a NULL pointer access in gfn_to_rmap() >> when userspace next tries to call KVM_RUN on the affected VCPU and kvm >> attempts to activate the new non-existent page table root. >> >> This happens since kvm only validates that cr3 points to a valid guest >> physical memory page when code *inside* the guest sets cr3. However, kvm >> currently trusts the userspace caller (e.g. QEMU) on the host machine to >> always supply a valid page table root, rather than properly validating >> it along with the rest of the reloaded guest state." >> >> http://sourceforge.net/tracker/?func=detail&atid=893831&aid=2687641&group_id=180599 >> >> >> Check for a valid cr3 address in kvm_arch_vcpu_ioctl_set_sregs, triple >> fault in case of failure. >> >> Signed-off-by: Marcelo Tosatti >> >> Index: kvm/arch/x86/kvm/x86.c >> =================================================================== >> --- kvm.orig/arch/x86/kvm/x86.c >> +++ kvm/arch/x86/kvm/x86.c >> @@ -3986,7 +3986,13 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct >> >> vcpu->arch.cr2 = sregs->cr2; >> mmu_reset_needed |= vcpu->arch.cr3 != sregs->cr3; >> - vcpu->arch.cr3 = sregs->cr3; >> + >> + down_read(&vcpu->kvm->slots_lock); >> + if (gfn_to_memslot(vcpu->kvm, sregs->cr3 >> PAGE_SHIFT)) >> + vcpu->arch.cr3 = sregs->cr3; >> + else >> + set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests); >> + up_read(&vcpu->kvm->slots_lock); >> >> kvm_set_cr8(vcpu, sregs->cr8); >> >> > > Isn't this self-defeating? If you drop slots_lock, cr3 may be invalid > again by the time you set cvpu->arch.cr3? > Uh, sorry, of course not. I misread down as up. Bad day for me. Will apply the patch. Still, don't we have a problem if userspace drops the memory slot where cr3 points to? -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.