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

Message-ID: <ZU51A3U6E3aZXayC@google.com>
Date:   Fri, 10 Nov 2023 10:22:59 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.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, 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, Nov 10, 2023, Xiaoyao Li wrote:
> On 11/6/2023 12:30 AM, Paolo Bonzini wrote:
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 68a144cb7dbc..a6de526c0426 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -589,8 +589,20 @@ struct kvm_memory_slot {
> >   	u32 flags;
> >   	short id;
> >   	u16 as_id;
> > +
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +	struct {
> > +		struct file __rcu *file;
> > +		pgoff_t pgoff;
> > +	} gmem;
> > +#endif
> >   };
> > +static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
> > +{
> > +	return slot && (slot->flags & KVM_MEM_GUEST_MEMFD);
> > +}
> > +
> 
> maybe we can move this block and ...
> 
> <snip>
> 
> > @@ -2355,6 +2379,30 @@ bool kvm_arch_pre_set_memory_attributes(struct kvm *kvm,
> >   					struct kvm_gfn_range *range);
> >   bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
> >   					 struct kvm_gfn_range *range);
> > +
> > +static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
> > +{
> > +	return IS_ENABLED(CONFIG_KVM_PRIVATE_MEM) &&
> > +	       kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
> > +}
> > +#else
> > +static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
> > +{
> > +	return false;
> > +}
> >   #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */
> 
> this block to Patch 18?

It would work, but my vote is to keep them here to minimize the changes to common
KVM code in the x86 enabling.  It's not a strong preference though.  Of course,
at this point, fiddling with this sort of thing is probably a bad idea in terms
of landing guest_memfd.

> > @@ -4844,6 +4875,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
> >   #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> >   	case KVM_CAP_MEMORY_ATTRIBUTES:
> >   		return kvm_supported_mem_attributes(kvm);
> > +#endif
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +	case KVM_CAP_GUEST_MEMFD:
> > +		return !kvm || kvm_arch_has_private_mem(kvm);
> >   #endif
> >   	default:
> >   		break;
> > @@ -5277,6 +5312,18 @@ static long kvm_vm_ioctl(struct file *filp,
> >   	case KVM_GET_STATS_FD:
> >   		r = kvm_vm_ioctl_get_stats_fd(kvm);
> >   		break;
> > +#ifdef CONFIG_KVM_PRIVATE_MEM
> > +	case KVM_CREATE_GUEST_MEMFD: {
> > +		struct kvm_create_guest_memfd guest_memfd;
> 
> Do we need a guard of below?
> 
> 		r = -EINVAL;
> 		if (!kvm_arch_has_private_mem(kvm))
> 			goto out;

Argh, yeah, that's weird since KVM_CAP_GUEST_MEMFD says "not supported" if the
VM doesn't support private memory.

Enforcing that would break guest_memfd_test.c though.  And having to create a
"special" VM just to test basic guest_memfd functionality would be quite
annoying.

So my vote is to do:

	case KVM_CAP_GUEST_MEMFD:
		return IS_ENABLED(CONFIG_KVM_PRIVATE_MEM);

There's no harm to KVM if userspace creates a file it can't use, and at some
point KVM will hopefully support guest_memfd irrespective of private memory.