linux-kernel - Re: [PATCH v10 4/8] KVM: mmu: Improve handling of non-refcounted pfns

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Fri, 23 Feb 2024 10:00:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: David Stevens <stevensd@...omium.org>, Yu Zhang <yu.c.zhang@...ux.intel.com>, 
	Isaku Yamahata <isaku.yamahata@...il.com>, Zhi Wang <zhi.wang.linux@...il.com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org
Subject: Re: [PATCH v10 4/8] KVM: mmu: Improve handling of non-refcounted pfns

On Fri, Feb 23, 2024, Paolo Bonzini wrote:
> On 2/21/24 08:25, David Stevens wrote:
> > +	} else if (!kfp->refcounted_page &&
> > +		   !kfp->guarded_by_mmu_notifier &&
> > +		   !allow_unsafe_mappings) {
> > +		r = -EFAULT;
> 
> Why is allow_unsafe_mappings desirable at all?

It's for use cases where memory is hidden from the kernel and managed by userspace,
e.g. where AWS uses /dev/mem (I think) to map guest memory.  From a kernel
perspective, that is unsafe because KVM won't do the right thing if userspace
unmaps memory while it is exposed to L2 via a pfn in vmcs02.

I suggested allow_unsafe_mappings as a way to make upstream KVM safe by default,
without completely breaking support for AWS and friends.