linux-kernel - Re: [PATCH 15/34] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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] [day] [month] [year] [list]

Message-ID: <aOUceqlAnsjQ8mo4@google.com>
Date: Tue, 7 Oct 2025 06:58:18 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Nikita Kalyazin <kalyazin@...zon.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Huacai Chen <chenhuacai@...nel.org>, 
	Michael Ellerman <mpe@...erman.id.au>, Anup Patel <anup@...infault.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, 
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Xu Yilun <yilun.xu@...el.com>, Chao Peng <chao.p.peng@...ux.intel.com>, 
	Fuad Tabba <tabba@...gle.com>, Jarkko Sakkinen <jarkko@...nel.org>, 
	Anish Moorthy <amoorthy@...gle.com>, David Matlack <dmatlack@...gle.com>, 
	Yu Zhang <yu.c.zhang@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	"Mickaël Salaün" <mic@...ikod.net>, Vlastimil Babka <vbabka@...e.cz>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>, 
	Maciej Szmigiero <mail@...iej.szmigiero.name>, David Hildenbrand <david@...hat.com>, 
	Quentin Perret <qperret@...gle.com>, Michael Roth <michael.roth@....com>, Wang <wei.w.wang@...el.com>, 
	Liam Merwick <liam.merwick@...cle.com>, Isaku Yamahata <isaku.yamahata@...il.com>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 15/34] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

On Fri, Oct 03, 2025, Nikita Kalyazin wrote:
> On 05/11/2023 16:30, Paolo Bonzini wrote:
> > From: Sean Christopherson <seanjc@...gle.com>
> > 
> > Introduce an ioctl(), KVM_CREATE_GUEST_MEMFD, to allow creating file-based
> > memory that is tied to a specific KVM virtual machine and whose primary
> > purpose is to serve guest memory.
> 
> ...
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index f1a575d39b3b..8f46d757a2c5 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> 
> ...
> 
> > -static int check_memory_region_flags(const struct kvm_userspace_memory_region2 *mem)
> > +static int check_memory_region_flags(struct kvm *kvm,
> > +				     const struct kvm_userspace_memory_region2 *mem)
> >   {
> >   	u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> > +	if (kvm_arch_has_private_mem(kvm))
> > +		valid_flags |= KVM_MEM_GUEST_MEMFD;
> > +
> > +	/* Dirty logging private memory is not currently supported. */
> > +	if (mem->flags & KVM_MEM_GUEST_MEMFD)
> > +		valid_flags &= ~KVM_MEM_LOG_DIRTY_PAGES;
> 
> I was wondering whether this restriction is still required at this stage or
> can be lifted in cases where the guest memory is accessible by the host.

Off the top of my head, I can't think of any reason why dirty logging wouldn't
work with guest_memfd for non-CoCo VMs.  We'd likely need to explicitly enumerate
support to userspace, and there might be some assumptions lurking in KVM, but
fundamentally it should Just Work (TM).

> Specifically, it would be useful to support differential memory snapshots
> based on dirty page tracking in Firecracker [1] or in live migration.  As an
> experiment, I removed the check and was able to produce a diff snapshot and
> restore a Firecracker VM from it.
> 
> [1] https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/snapshot-support.md#creating-diff-snapshots
> 
> > +
> >   #ifdef __KVM_HAVE_READONLY_MEM
> >   	valid_flags |= KVM_MEM_READONLY;
> >   #endif
> > @@ -2018,7 +2029,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >   	int as_id, id;
> >   	int r;
> > -	r = check_memory_region_flags(mem);
> > +	r = check_memory_region_flags(kvm, mem);
> >   	if (r)
> >   		return r;