linux-kernel - Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

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]

Message-ID: <ZUPFmwWjPFlXRlPi@google.com>
Date:   Thu, 2 Nov 2023 08:51:55 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Kai Huang <kai.huang@...el.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "brauner@...nel.org" <brauner@...nel.org>,
        "oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
        "chenhuacai@...nel.org" <chenhuacai@...nel.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "willy@...radead.org" <willy@...radead.org>,
        "anup@...infault.org" <anup@...infault.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        "mic@...ikod.net" <mic@...ikod.net>,
        "liam.merwick@...cle.com" <liam.merwick@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>,
        "tabba@...gle.com" <tabba@...gle.com>,
        "amoorthy@...gle.com" <amoorthy@...gle.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "michael.roth@....com" <michael.roth@....com>,
        "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        Vishal Annapurve <vannapurve@...gle.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "mail@...iej.szmigiero.name" <mail@...iej.szmigiero.name>,
        "yu.c.zhang@...ux.intel.com" <yu.c.zhang@...ux.intel.com>,
        "qperret@...gle.com" <qperret@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        Yilun Xu <yilun.xu@...el.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "ackerleytng@...gle.com" <ackerleytng@...gle.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Wei W Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace

On Thu, Nov 02, 2023, Xiaoyao Li wrote:
> On 11/2/2023 1:36 AM, Sean Christopherson wrote:
> > > KVM_CAP_MEMORY_FAULT_INFO is x86 only, is it better to put this function to
> > > <asm/kvm_host.h>?
> > I'd prefer to keep it in generic code, as it's highly likely to end up there
> > sooner than later.  There's a known use case for ARM (exit to userspace on missing
> > userspace mapping[*]), and I'm guessing pKVM (also ARM) will also utilize this API.
> > 
> > [*]https://lore.kernel.org/all/20230908222905.1321305-8-amoorthy@google.com
> 
> I wonder how this CAP is supposed to be checked in userspace, for guest
> memfd case? 

It's basically useless for guest_memfd.

> 	if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO) &&
> 	    run->exit_reason == KVM_EXIT_MEMORY_FAULT)
> 		abort("unexpected KVM_EXIT_MEMORY_FAULT");
> 
> In my implementation of QEMU patches, I find it's unnecessary. When
> userspace gets an exit with KVM_EXIT_MEMORY_FAULT, it implies
> "KVM_CAP_MEMORY_FAULT_INFO".
> 
> So I don't see how it is necessary in this series. Whether it's necessary or
> not for [*], I don't have the answer but we can leave the discussion to that
> patch series.

It's not strictly necessary there either.

However, Oliver felt (and presumably still feels) quite strongly, and I agree,
that neither reporting extra information shouldn't be tightly coupled to
KVM_CAP_EXIT_ON_MISSING or KVM_CAP_GUEST_MEMFD.

E.g. if userspace develops a "standalone" use case for KVM_CAP_MEMORY_FAULT_INFO,
userspace should be able to check for support without having to take a dependency
on KVM_CAP_GUEST_MEMFD, especially since because KVM_CAP_GUEST_MEMFD may not be
supported, i.e. userspace should be able to do:

	if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO))
		abort("KVM_CAP_MEMORY_FAULT_INFO required for fancy feature XYZ");